Where can I find the mini-expansions for Carcassonne? I've seen there are several such as "The Cathars" and "Crop Circles" but don't know where to get them, my local board game store doesn't have them.
First off, the expansions you have listed are official. Depending on which expansion you're trying to find (and which print run) may be a little challenging:
The Cathars (BGG) (German: Die Katharer) was originally released in the German magazine "Spielbox", and then again in the set "Der Carcassonne Almanach". It was re-released in the US as part of the "Cult, Siege and Creativity" set (although it was renamed to the "Siege" in the set, explaining why you may have had a hard time finding it, and apparently the artwork was altered for the US release as well).
Starting in 2010, the Crop Circles (BGG) (German: Die Kornkreise) can be found in German versions of the base game. I'm guessing it will be released in an anthology set (like "Cult, Siege and Creativity") at some point, but that's speculation on my part.
You can buy them at various online stores. A look at the boardgamegeek.com entries for them will lead you quickly to some available retailers, as well as ebay sales of them. Further, BGG has a sales system which allows gamers to sell used supplements. Many other online sources are available, as well, and many of those advertise on BGG.
Here's a partial list with the BGG links...
You can purchase most of the official expansions, mini-expansions and replacement parts (individual meeples, tiles, etc.) at http://www.cundco.de/ ... it's all just in German though! :(