Thunderbird has it, even though it’s not always listed in their
catalog. When I bought some, a year or two ago, the number for
tufa was 612103, and it was $1.40/lb. It comes in chunks.
Thunderbird is at 1907 W. 66 Ave., Gallup, NM 87301,
505-722-4323, or 1-800-545-7968 for orders. Their minimum for
orders at the time was $30. Good luck.