I ran into the same problem. I had always used Homasote, but there was none to be found around my area other than special order at about $30 a sheet.
So I switched.
I started using Temple-Inland's QuietBrace from Home Depot. I used it three years ago to build my first first O-Gauge layout, and I am using it now for my presently under-construction layout. Its price has gone up in the past few weeks, and the price for three sheets I bought today (for a total of 21 sheets, I believe) was $9.29 / sheet.
I cut it with a jigsaw and knife-edge blades or just with a box-cutter knife, which is how I used to cut Homasote - dust free!
I believe it is very similar to Homasote. If in doubt, buy a sheet and try it. I used to paint it before I used it, but have left the last several sheets the natural black color. The cut edge looks like wood.
You may want to take a look at my layout thread - there are some pictures that show the QB painted and unpainted: Alex's Layout