I agree with philb - any local fair will have thousands of covers which you can choose from. Here in the Uk I go to a number of fairs and have acquired some really nice material at a fraction of the prices offered on
ebay. You might also try your local charity shops for some interesting covers. Oxfam often get bequeathed collections and unless they are mainstream then they offer them up at 99p (or 99c) a cover. My local shop currently has over 100 George V postcards at 99p a go. Lots of them have really great postmarks on them, including a fabulous squared circle I picked up last week.
ebay is generally full of people trying to make a profit on stuff they have picked up cheap - I should know, I put stuff on there in the winter when it's cold and dark! The question is, where did they get them? Wherever it is, you too can get them there as well.