I've been down that route too, but the loops available aren't anything special, or just not the kind of thing I'm after.
I tried a copy of "Acid Rock" purely for its copyright free loop CD rom. It was cheap and has a load of decent drum patterns. The dedicated copyright free sample CD's tend to be expensive.
I don't want "standard" rock or any other kind of beat, so when I have written something, I tend to have the rythm or drum pattern, and time changes, in my head. I sit down with the drum machine and guitar and basically mess about, playing with various ideas, trying to create a drum pattern that sounds passable and fits in with the riff/chord ideas I have in my head. I admit, its a time consuming ball-ache of a job, but if you end up with a "unique" drum pattern for your song, then it's time well spent.
I have spent days on one drum pattern before now, but sometimes when that happens, you are better off walking away from it and having a think (often best with guitar in hands! :wink: ) about the song as a whole and what you are aiming to acheive. Also play about with the BPM count, it doesn't always have to be 120BPM. I have some that are about 70BPM and a few in between, but they fit the idea and feel of the song.
Get a basic pattern you can work with down, play about with it / jam it around, and then get critical with it, pull it apart and reconstruct it and other patterns so that they fit. Re-focus on the "big picture" of your song, and above all, tell yourself that its going to be a cracking song when you've finished!