I would suggest a few things for this cat.
1) Stop feeding her canned salmon and start feeding her canned kitten food (unless I've misinterpreted and you're feeding her canned kitten food in salmon flavor). It's better for her and it won't change your routine at all (other than a trip to the grocery/pet store). I know that Iams kitten food is decent and usually readily available.
2) Do some internet searches and use the links that Katie sent you to look for low cost/ no cost clinics. If there don't seem to be any available to you, start calling all the vet's in your area. Explain your situation. That she's not your cat, she's a stray, she's been having litter after litter, and can they work with you to provide a low cost or no cost spay. Don't be afraid to call several times if you have to leave messages or if no one calls you back in a timely manner.
3) Consider having her spayed now. She may not be as far along as you think she is. When we took in a stray momma cat this Oct. I was sure she was ready to pop! She looked huge! (but then I'd never seen a pregnant cat before!) When I took her to the vet, I was given a range of 2 to 4 more weeks before she was due. I figured this vet just didn't know what she was talking about. There was no way our little Chalupa was going to wait that long for her babies. Well, sure enough, about 3 1/2 weeks later, she gave birth! I think if I had really understood that when we took her in she was approaching the halfway mark, instead of ready to give birth that night, we may have had her spayed then to end the pregnancy. Obviously, this is your choice, I'm not saying you should do this, but I want you to know you have the option.
4) If cost is a factor, talk to your roommates and neighbors about going in together to get her spayed. Money can be left at the vet of your choice if people are uncomfortable giving it directly to you.
5) If NONE of this seems possible to you, then I would start looking for a foster home for her now. You can use http://www.petfinder.com
, then click to find your city, or the nearest city to you. Even if you don't have any organized resources available to you, I'm sure that there are people who do things like this on their own, and they may be able to help you out. Other members here can help you out with ways to screen potential homes for her.
Good luck to you and her, however you decide to continue. Thank you for caring about her welfare.