I went for M520's when switching from flats to clipless, but went with the 'normal' release cleats rather than the multi-release ones. No particular reason, just the pedals came with them and the shop I bought from didn't have the multi's in stock. So far I've only good things to say about them, and with the tension loosened right off I find it's easy to clip in, easy to clip out, I can pull up on the pedal pretty hard when riding and have been able to yank my foot free in the few "Oh god, I really need to get my foot down NOW!" moments I've had.
As for where to buy them, I got mine from Go Outdoors in the sales and they cost me about £15, but you can get them at a lot of online stores for that sort of price anyway.
CRC have them for £15.99 which is the cheapest I've seen lately, but then I've not really been looking. You'll also (probably) need different shoes then you would for the cleats you mentioned in your first post (worth considering if you've already invested in some cycling shoes), but they can cost as much or as little as you want, running from twenty-odd quid right through to hundreds of pounds. Again, mine came from Go Outdoors in the sales and cost me £45ish. I'd say it's been the best money I've spent on the bike so far and now have trouble going back to flat pedals.