What kind of filter fabric should I use for the curtain drain in my crawlspace?
For the first part, as it's in answer form: 

Many hardware stores that sell perforated pipe also sell filter fabric already made into a tube that neatly fits over the perforated pipe; if it's a warehouse store (not one where you get a ticket and then go out to a lumber yard), it'll likely be kept near the perforated pipe.

As for the chance of clogging -- I can only assume it depends on what sort of soil you have.  If lots of soil starts washing into your pipe, the ground level might fall slightly, but I personally haven't seen too much problem, but I don't have high-clay or silt content above my drain.  (I do have clay deeper down, however, but you're going to have other problems if you're trying to drain through clay)