In general (since I don't know specifically about your Civic) they can be either difficult or easy. Some can be removed (often with a special but not too expensive puller) while still on the engine, that makes it easy to do. All you have to remove is the belt.
The rest are different degrees of difficulty. Most of them can be removed by only removing the compressor from the engine mount (but with the refrigerant lines still connected). The really difficult ones require you to remove the compressor completely from the engine compartment.
As for availability, that varies too. Some makes/brands only sell the compressor and clutch as a set. Others are available separately. I did a quick search on Rock Auto and it shows just the clutch for a 2000 Civic, so you might be OK there.