Try Arizona Cactus Garden for seed

Here is a link that might be useful: Arizona Cactus Garden

M. luethyi is not offered as seeds yet! And when seeds will be offered within some years, they will be expensive as the fruits stay in the plant body (like M. theresae). Here in Europe, the grafted plants are dropping in prices the last couple of years. Three years ago, you still payed about 20 euro for a small grafted plant. Now you can find these for about Â3. And some nurseries here are offering them as rooted plants as well now (but still very expensive). Rooting this species is not very difficult. About as easy as M. theresae.
Pieter