Safe way to firm up stools?

I have 3 cats.
Two of them will be 8 years old in Sept. The other will be 6 years in July.
In the past couple of years they have all had a problem maintaining firm stools unless they are on a very low fiber diet. And by that I mean it's pretty much constant diarrhea.
They do well on wet food, and I've tried a low fiber dry food since Saturday which seems to be working so far (2% max fiber).
Though in my area I am limited to only 3 foods that meet this requirement.
Science Diet
Purina One
Pro Plan
I'm not a big fan of any of these foods.
I tried my local store which sells better products but the lowest fiber any of those had was 2.7 to 3.0 max, which is still too much.
I'm going to search online, to see if there is anything on there which meet the requirements and that I can afford.
Does anyone know of anything I can safely give my cats each day to help firm up their stools on a higher fiber diet?

My cats are on Purina Sensitive Systems for the same problem, mushy stools. I buy it at the grocery store, they seem to be doing fine on it.

You can try giving your cat a pet psyllium fiber supplement such as Vetasyl (available at pet stores).
You didn't say if your cat has been to the vet or not. Metabolic disease such as hormonal changes, endocrine dysfunction, diabetes and liver disease can cause diarrhea. In addition, primary diseases of the bowel such as inflammatory bowel disease can also show up as a loose stool.

Yes, be sure there is no medical reason for this first. You can feed a teaspoon of pumpkin - not the pie filling. I would also be giving them some yogurt as well to help sustain the good bacteria.

My cats are on Purina ONE Chicken & Rice as of last Saturday. As I said before I'm not a big fan of that food, or any of the Purina formulas really.
While their poop has gotten a little stinker (as is normal with my gang and dry food) their stool still remains nicely formed.
Usually if a food will not work for them I will see a change with in 12 hours.
I will look into that Vetasyl, can't say I've heard of it off hand. All of my cats have been to the vet on a regular basis and for any strange issues that have come up. Including Tage and Isha whom I did have a unknown and undiagnosed problem with them. They both went to 2 different vets each, both put random treatments, had lots of tests done but all in all nothing was really revealed most everything looked good. Eventually through a change in diet (or perhaps time) their problems subsided and have not returned.
I am a freak about their health and their food (which is why I so badly want to get them back onto a better food.) I know the last thing they need is constant diarrhea as I worry that would bring on the start of another relapse of their previous problems.
Suebgone: I have considered the pumpkin but my cats are largely not interested in it.
The yogurt I might try however. I certainly want to try and go a natural route as much as possible. I've used it with dogs but personally have never seen any change health wise at all.
On the bright side, I have found 1 higher quality food which has the necessary fiber content.
Lick Your Chops - Lamb
With a few other brands coming in close with a fiber percentage around 2.5-2.9
However I have no place locally that is a reliable source to buy this food.
I know petfooddirect carries it, but I'm going to look for other online places as well in case they are ever out of stock.

A vet friend said that she has had excellent results with cats with diarrhea, using Eukanuba's Low Residue.

Thank you for the suggestion but I will not feed Eukanuba.

They get diarrhea from high fiber foods, not the other way around? Usually fiber is used to firm up stools. That's one of the advantages of pumpkin, that it's very fibrous. Other things you could try is mixing the pumpkin with their wet food, or some really overcooked rice. Carrots have a similar effect to the pumpkin, except I think they have more sugar, but you could cook them in some chicken broth and then mush up.
But it really sounds to me like they need more fiber, not less...

I think my cats are just very specific in their dietary needs.
One of my current dogs, when younger could only have a max of 3% fiber before her stools started to get a bit too loose, but now she is ok with pretty much any normal range.
Typically you are right higher fiber foods will help to regular and firm up the stool, it's just been shown that what that "normal" level is for my cats is slightly less then the average cat (at 2% rather then a typical 4%).
With out fail for the past 2 years, this has been the story with my cats, I've seen it ring true no matter what kind of food or formula I've had them on, home made, raw, canned and dry.
I am most interested in studying the dry food more, to see if there is perhaps a common ingredient in them that just no longer agree with my cats, it might not be the amount of fiber but rather the type of fiber that is effecting this.
On raw food and canned food their stools are hard and nicely formed (sometimes too hard).
I have had success using Benefiber with dogs to firm up their stools, I've never tried it with cats as I've only had the orange flavor (my cats won't touch that.)
But now that they have a flavorless formula I might see how that works with my cats.
A very over weight foster dog I had once could not loose weight on "lite" dry foods because they had excessive fiber, it simply ended up keeping the weight on, once I switched to a low fiber diet and added in the benefiber it worked like a charm all around.
If you want to read some information on fiber this is a really great link.

I don't have any suggestions as my cats tend to have the opposite problem. They are prone to constipation. But I do sprinkle fiber in their food for the constipation. I think fiber regulates, whether dealing with one or the other.