If you had a piece of shit Chevy, any old tires would do and all 4 wouldn't need to even match, but you don't and you need good tires and good tires aren't cheap. Hopefully this will help.
New, used... here is a list of vendors of parts. Many t have websites and if they do they are linked from here. Where they have email addresses they are listed here, too.
The net is one of the best places to find a used Benz; I've found both of mine on the net. Many newspapers now publish their classified ads on the net, and and there are some extremely good non- newspaper classified services with plenty of cars.