I got mine on Newegg.com, They get them in fairly often.
You can crossfire with a 5870 of any type [VRAM], a 5970, and I'm pretty sure, but not certain that you can do a 5850 of any type as well [VRAM].
I do suggest getting a 5870 with a 2 GB frame buffer though, because if you get one with 1 GB to Crossfire with it, you'll be stuck with only 1 GB frame buffer space through out the configuration. But really, it seems cheaper to just buy 2 E6 Editions, instead of getting a E6 and a Toxic 2GB card.
I just finished a week of researching this very situation... and yeah, anything in the 5xxx series, i.e. 5850/5870/5870 2gb/5870 e6 will Xfire.
And he's right about the RAM. Get the 2GB so you'll have the extra headroom for eyefinity...