Flori,
The only difference between Columbia Excelso is that Columbia Supremo is bigger beans, which doesn’t affect the taste. Excelso and Supremo will vary in taste from each other, and from their own crops, because the beans come from various farms and various crop yields.
Do you have a local roaster where you buy your coffee? You'd probably be able to find some Colombia Excelso or Supremo there. You probably won't be able to find both of them at the same time, so you may end up trying one first and the other later.
Every roaster sets his/her own prices. You may want to start out with a half pound at a time.
Rose