Here's what I know...
This source advertised on the Carving List:
elarairah@aol.com
phone: 888-644-8779 (S. Central PA)
3 sizes in stock (this was 3/24)
4" by 4" by 1/2 ", weight 6 oz. $2.60
2 1/2 by 5 by 3/4 weight 8 oz $4.50
4 by 6 by 3/4 weight 14 oz. ($?)
And Mars Staedtler has a number (for info on location
of retailers: 1-800-776-5544

I've had one block of UW (now MarsCarve) and think the
advantages for letterboxing are the sizes available, that
it's easy to carve ~ soft but not marshmallowy (how's that
for esoteric?) and quite satisfactory for most efforts,
and it's thick enough that the block will serve as a mount.
The disadvantages are that one will likely have difficulty
carving very detailed stamps (like small text ...) and
the printing can get kind of squooshy (fine lines squish,
outside edges may print, that kinda thing ... ) because it is
softer than Mars and other like materials.
Deborah