I've always liked Gracilaria since they're non-demanding, don't go "sexual" as often as Caulerpera, and are great Tang food. Ulva sp. is also very attractive and grows really fast.
Sorry: can't help you about where to get them locally.
i got gracilaria and ulva from ipsf.com in the '9 for $99' special and I still have it 3 1/2 mos later! It was in my main tank but my tangs sort of ate most of it so I put it in its own fuge to 'recoup.' It seems to be growing slowly but nicely under a coralife 50/50 bulb. my tangs get nori and an occasional treat from the fuge.
