Try the Luv My Pet clinics through Petco. They are held on weekends at the various ones in the area (you can do a search based on your area and it will tell you where/when). Be advised, it's not by appointment, so you may have to wait a bit. But, it's definitely cheap (looks like rabies shots are $15). Here's the website: http://www.luvmypet.com/
Also check with Banfield, the vet inside of PetSmart. I know they used to have a couple of afternoons a week where they'd have shot clinics (and you wouldn't have to pay the office visit fee), so it would be worth it to see if they still do. GL!