Our local Home Depot carries homosote board. I haven't had any trouble getting it. Most people prefer it because you can drive spikes into it easily, and they hold.
I've had no trouble using cork, if the spike can contact the subroadbed (which, in my case, is plywood).
I'm getting ready to build a new layout. I handlay my track and have always used homasote, which although getting harder to find, is still available. Just curious...has anyone tried handlaying track on any other type of material and, if so, what are the pros and cons?