If you don't mind painting from a photo, I'd suggest checking out MMAweekly's photo gallery. They have great images, and pretty high-res. In terms of photos where the fighters are posing, go into any of the events and then click on "weigh-ins".
If you finish anything, you should send a scan to the fighter. A lot of times they put good artwork people have done of them up on their websites, and you'd get the credit for it, of course.
http://69.46.29.2:8080/absoluteig/ga...ategoryid=1098