We have all done it at one time or another. Not all of our finds were from a flea market or yard sales. The most reasonably priced finds undoubtedly are the flea market or yard sale variety, but there comes a time when we need to fill a gap and have to look to other collectors to fill that need. 

When that happens, where do you go? Most advanced collectors are well organized and known to others as having a variety of a certain type of collection. Others are a hodge podge of just about everything.

Then there are those who trade and deal regularly and will provide what you are looking for "for a price" With this type of collector, you can count on paying top dollar, but you will get what you are looking for.

Trading is sometimes the best answer and is done frequently among advanced collectors. Museums seldom engage in trading except with another museum