Lareefs.com has a large selection of macros and some rarer varieties, too. On the downside, they are overpriced, considering the size of the product (frag-sized). Live-plants.com has a very nice selection as well, and are much more affordable (same price as lareefs, but instead of a frag you get a pint). They also have some species which lareefs don't carry, but the converse is true as well. reefs2go.com has a decent selection as well, although not as many as lareefs or live-plants. Reefcleaners.com also has macros; a smaller selection than any of the above, but they offer one species which I haven't seen anywhere else; a brown branching Dictyota which they say will not overrun your tank like others of its genus. eBay often has macros sold as individuals or "refugium packs"; mostly Graciarella and chaeto, but every once and a while you might strike gold. For information on macros in general, I recommend giving marineplantbook.com a look; it has information on the requirements of many species/genus regularly found in the trade, and also has pictures and descriptions of everything, so apart from an info source, it is also helpful for IDing macros.
As for recommendations, I would suggest the genus Botryocladia ("red grape macro algae"), dragon's tongue (genus Halymenia), and Codium Lucasii (rather unimpressive at first sight, but if you look a little closer you will notice a beautiful blue/green iridescence). They are some of my personal favorites. But most of all, keep an open mind and be creative: seemingly unattrictive macro algae sometimes turn out to be beautiful additions to a display tank. Good luck! And post pictures, ok?