Our local Walmarts, Kmarts, and ToysRus all have full aisles for diecast cars. However, its difficult to find models prior to the 60's, and most are in the sports car category. Most are at full retail this time of year, but you may see some clearance sales in August. As Paul Norton mentioned, the best time is after Christmas at Walmart. Cars will be as low as $3 to $5. I let my five year old play with them first. Once they are off the favorite list, or broken, they go outside.
If you need 18 wheelers, ToysRus carries a full line that also includes flat beds and tankers. They list for $15.
You can find plenty of older period vehicles, but you will probably have to buy them online.
Paul