I see them all the time in the wild.
Whole Foods, airports and gyms are where you will see them.
I don't think there's an "appropriate" way to approach them, but I've had success approaching them by not mentioning their muscles (the bigger their arms, the harder it is to do) and not acting like a weirdo.
I've met Melissa Frabbiele, Christina Sondgroth, and Allison Ethier in the wild, plus some whose names i don't know. I had pleasant conversations with all of them. Christina asked me if I wanted to take a picture with her (I did). Allison is now a Facebook friend, although she and I haven't said all that much to each other. It's not that hard, just be a gentleman and treat them like any other women you'd meet.
Also Rebecca Hasulak, but she is not a bodybuilder, she's a model who's very fit.
I know it can be difficult, but as it's been said a few times above you just go with the flow and treat them like any other person, because despite looking different, most are really just people, although I've met a few who are a little nuts.
I think you can start with a compliment on their physique. I haven't met a woman who didn't like a compliment. In my experience, women who work hard and spend so much time working of their body really like a when someone notices and has something kind to say. Many times, depending the location and situation it leads to a real conversation. But again don't be creepy and overly admiring. You've said it once, she knows you like her look, you don't have to say it again in a different way.
PaulMasvidal (above) knows where to find them. I think in some ways the gym is the most difficult place to strike up a conversation, because I know myself and the other hardcore guys and girls in my gym don't really like to have a conversation on workout time. Paul's also spot on about Whole Foods and Airports, at least in my neck of the woods. Almost every other time I'm in Whole Foods in my town I run into someone who body builds, male or female. And airports here in south Florida too.
Funny story about the airport, I was always into muscular women, way before I got into bodybuilding myself, and I was at the baggage claim carousel when I saw a gorgeous (in my eyes, at least) woman bodybuilder waiting for her bag. She was probably in her early to mid 40's and I was just in my early 30's. I was coming home from a trip and she was from out of town meeting girl friends. I didn't know her from Eve, but ended up driving her to her hotel on the beach and all I did to start the conversation with her was to ask her "Do you need me to get that for you?" as she reached for her big bag. She said "No, I got it" and as she was pulling it off with one arm she gave me a quick flex with the other. We got to talking more after that, I won't bore you with the rest, I just wanted to show how really easy it is, if you just relax, breath and don't over think what you are going to say.
Damn I hope I can finally get a more stable job and treat myself to one-off visits a couple of times a month and find myself a place with above-average women...
ive had quite a few encounters with women at a few gyms, running the gamut to "that was probably real weird" to "i asked her out and her dad is scary" to "we are good friends and she has a great body but I won't obsess over it". Never really had the pleasure of an "in the wild" experience (don't really consider a gym the wild lol) but my experiences were much more pleasant the less i made a big deal out of it. the more i let the fetish drive the more awkward i felt which meant the whole interaction was going pear-shaped.
don't ask if they need a spot lol