Do you know what Tai-Chi is? It's that slow meditative martial art. You're more likely to find old guys in there. Look, you're talking about finding a girl who is not extremely heavily muscled, but with some decent tone, right? To me, this says "cardio" and/or health nut. When it comes to martial arts, etc - why not find a girl who is into boxing or karate or something a lot more active?? Here's a list of suggestions off the top of my head:
1. Health food store (organic food etc)
2. Supplement store (GNC et al)
3. The gym - eventually, you WILL see one on the treadmills, guaranteed
4. General public areas - college campuses, stores etc - like Chainer's post suggested, the law of large numbers will eventually work out in your favor. At least 1% of girls out there have some definition in their shoulders and arms. Maybe more.
5. Athletic events - basketball, baseball, football games - someone in the stands will be buff by the same principle
6. Join a running/jogging club - or a biking club! CARDIO!!! Maybe take a spin class at the gym or any kind of organized class at a gym - yoga classes and most aerobics classes are typically included in your membership, and are usually dominated by women (90% or more depending on the class)
7. The internet - the ultimate large population pool if you know how to work it.
8. Public parks - playing tennis perhaps, jogging, or walking a dog perhaps?
9. Collegiate gymnastics meets or other female collegiate sporting events - only drawback is they'll be on the defensive because they're not in hook-up mode at that time...