diarrhea in stray - causes?

I have a stray mother cat living with me, she has had chronic diarrhea from before I got her - 2 months now. She's been wormed, I've tried dietary changes, and she's currently on a course of flagyl which seems to be of limited success. My big worry is that her kittens, which have been fine up till now, have just started with diarrhea - they're 12 weeks old. Does this suggest a cause to any experienced cat rescuers out there? I'm in real despair because I homed one of the kittens over a week ago, and they've just called to complain that she has bad diarrhea. I might end up having to take her back. So there is something contagious going on, and I'm worried that it's more than just a tummy bug because of the fact that the mother has it for so long.
I've just called 2 vets to ask about the price of a testing the mother cat for FeLV, and it's 90 euros - I can't afford it.
Just hoping these symptoms might ring a bell with someone out there. The mother cat is small and thin, sheds a lot of hair, has very dry skin on her paws, and is not terribly energetic. Her rear end is dirty and she smells gassy. On the other hand she has put on weight since I got her, has fed her 3 kittens up to 11 weeks. All had a runny eye, sneezy catflu virus when I got them, but kittens had no diarrhea till now.
I don't know whether to keep hanging on, and hope it clears - I'm worried about the safety of my own cats too.

Welcome to the forum, planetx!

There's no quick and easy answer for what might be causing her diarrhea.

First and foremost, though, is to ensure that she has access to the simplest of diets with lots of water (cats are, by nature, water-challenged at the best of times, and diarrhea can result in rapid dehydration). I think I'd be trying a watery mix of rice and chicken in the interim...I'd probably also throw in some probiotics - none of this will cause any further damage, and might at least stabilize things in the short term. I'd be especially worried about the kittens - they can dehydrate in what seems like a flash!

In terms of Veterinary care, is/are there animal rescue groups in your general area - RSPCA's/smaller organizations? If so, I'd be approaching them for assistance...it certainly would be in their interest to help by keeping this group of cats in the community versus having you leave the cats on their "doorsteps"...

There's another section on TheCatSite Ferals and Rescue" where your post might garner the help of others more knowledgeable...you'll find this part "Caring for Strays and Ferals" right here http://www.thecatsite.com/forums/forumdisplay.php?f=9

I've sent you a private message as well, as I sometimes find myself in similar circumstances.

thanks whiteshadow, I'll try your suggestion on the pm. I was just hoping there'd be someone who'd say - hey my cat had the same and it was XXX. Flagyl hasn't worked at all, and I really don't believe it's being caused by diet. All the vet can say is that it is probably viral. I go online and look up all sorts of cat viruses, and start to get really scared for my own cats. They have already been ill with a respiratory infection from the strays, despite being up to date on vaccinations.
The real problem is that I've had to stop the whole rehoming effort, and it's such a shame - this is the peak age for kitten cuteness, they're growing so fast.