ebay is always good, although there are a lot more 1:24. Also check places like Zellers,Peavey Mart, True Value hardware (if you have one near) even sometimes places like Superstore, Extra Foods will have some, usually quite cheap.
If you eventually want to get into more expensive ones, places like
http://www.collectablediecast.com/
http://www.diecastmodelswholesale.co...let/StoreFront
http://www.3000toys.com/
If you do a google search for collectible diecast you will find hundreds of places to buy them.
good luck with your collection!
*edit* check this link, think you'll like it
http://www.m3tech4toys.com/inc/sdetail/963