I would suggest making it an official non-profit, simply for legal matters, taxes, and any other issues you may run into after a while. There's an article on how to start charity organizations in a magazine I read the other day. But it's my "bathtime book", so I'll get it tomorrow after my bath and bring it down to the computer and post the info for ya.
You can try a jar. My group tried putting a jar in a coffee shop (one volunteer is good friends with the owner/barista), but we got maybe $10 in a whole week. We also have a jar at the vet we use, and it's got maybe $15 in 6 months of being there. Not much, but I guess every little bit helps.
You could also do fund raisers like bake sales, or sell beaded bracelets, or whatever you can think of. We sold cookie dough from a local company...I think it was $20 for a pound of dough, and we got $15 of it ($5 went to the company for their expenses of supplying it).
Be sure you get any permits needed, for selling stuff! I've heard several stories of KIDS being fined for having lemonade stands without a business license! But hey, that's the world these days...
I don't really get involved in the money part. Like you, I don't work and can't afford to donate to all these things. However, I do go to the sanctuary to clean the place and care for the cats (one cat that needs medicine won't let anybody but me touch her!) about 3 or 4 times a week. Our funds aren't enough to hire paid people, and it seems lots of people don't want to volunteer when it requires physical labor. (lots of times we get people that come and just play around with the kitties, and I end up doing all the chores).