You should be able to get it at your local paint shop, that's where I always got mine. I'm sure not all places carry it though, so you may have to make a visit or two. They should be able to order it if they don't have it on the shelf too.
As far as sanding it, it is harder than Bondo, but I've found that while it wears out the grit on the sandpaper faster it doesn't "clog" the sandpaper as quickly as the softer bondo material. It also "feathers" better on the edges, again because its harder it doesn't "chunk" as badly as Bondo does.
Working it is a little different, but I wouldn't say harder. And its more than worth it for the results, smoother edges and being impervious to soaking up water are both HUGE benefits.