Learning Is Fun, on Menaul near San Mateo is good, especially for workbooks, but it seems their prices have gone up a bit. Also, Barnes & Noble in Coronado mall has them for reasonable prices.
I find out about stuff through my HS friends, then I find it on-line and end up buying a lot of stuff through Amazon (good prices, often free shipping). I borrow lots from the library too.
You can also find free games and printable stuff on various web sites, just search for "math worksheets" or "math games" etc.
Title Wave is good too, especially if you want text books.