This topic came up about six months ago -- yes, AMI instant roadbed is no longer made. Woodland Scenics foam roadbed and all of the brands of track with integrated roadbed probably did it in more than anything else.
I found it to be rather useless as actual roadbed because it was nowhere near the right shape, but by stretching it sideways and painting it to look like asphalt, it can be used to make highways and parking lots.
You might still be able to find some left over if you check local hobby shops.