There's tons of drum libraries, I'm sure you can get pre made drum loops from those. Any online store will have em, and I'm sure there's smaller scale freebie ones, though most of those come when you buy other sample sound packs.

Or you can use some drum machine software (other people will have recommendations, I've never used any). That'd let you make up your own entire drum track from scratch.

You want to create a waltz? Rock 'n roll waltz? What about Walter Lance? He had a woodpecker.

Not a redhead
Mx. Remy Ann David

You're going to have to spend some money:

http://betamonkeymusic.com/Store.html

OOps. Sorry. Here's the link for instant download:

http://www.drumwerks.com/store.html

You might search iTunes for Dino Drums if you're just looking for a short clip to loop. His clips usually sound great, but they're just short clips you would have to loop. - which should be right up your alley using Acid. You'll have to do the WAV conversion on your end.

Listen to the one titled "Ballad 6/8 70 Wet", it's the closest thing to the Metallica song you refer to.

There are numerous drum track vendors on iTunes to choose from - some better sounding than others.

Thanks all. Will search suggested links.