It might also take quite a bit of space and time to grow on a large number of seedlings to a stage where you can tell whether they have the colour, growth habit and size that you want.
Doncaster, South Yorkshire. Soil type: sandy, well-drained
I doubt anyone bothers selling seeds of JM cultivars because there's almost no market for them. Given you won't get the same cultivar from the seed very few people would be interested in paying a premium for seeds that have come off a specific cultivar when you'd get almost the same results from seeds from the species tree.
Most Acers are grafted to a rootstock, in the same way that fruit trees are grown. The rootstock determines the vigour and eventual size of the tree and the graft determines the characters of the "top". A specimen such as sango kako, has been specially bred and selected. As others have said it is unlikely to come true from seed even if you can get some.
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The answer on the below link goes some way to answering your question but I think it's the commercial awareness rather than the scientific understanding that you've got stuck on.
https://www.houzz.com/discussions/3887787/why-are-japanese-maples-not-true-to-seed