Diecastdirect has lots of 1:43 and 1:50 vehicles over a large price range and provides good service. So does Mint Models and so does Buffalo Road (constructionscalemodels.com), and there are others that specialize. eautomobile has a vast selection many of which you will not find anywhere else but is generally not as low priced as other sites.
However, the largest selection I have found, by far, particularly at the lower end of the price range (below $20) is on Amazon.com. It helps to use their search engine and to keep in mind that it searches in a different way (more thoroughly) when restricted to only one department at a time. That said, if you go to the site and set the department selection at the top of their page to "All Departments" and then type "1:43 Amer Hobbies" (use the plural) into their search box and hit enter, it brings about 150 cars priced at either $9.90 or $11.90 offered by a quirky supplier out of Poland that offers only this small selection - prices are low and delivery takes about a week.
However, if you select Toys and Games department, and then use of their search engine for "1:43 automobile" for example, it will bring up about 400 pages of 24 cars each entries priced from $5.90 to $400+. "1:43 truck" or "1:50" truck" will bring up thousands more. I generally search by year (1:43 1951 for example, or specific (1:43 1953 Mack B61 truck"). If in this Toys and Game section, you search for 1:43 brands known to be inexpensive (New Ray, Yat Ming) you will find a lot: "1:43 Yat Ming" in Toys and Games brings up about 250 entries piced from $6.
