Any better nursery or garden center should carry a variety. Roger's Gardens in Corona del Mar has a good selection, including many less common cultivars. But you should know that "exotic" and "reasonable price" are not necessarily compatable terms when it comes to JM's :-) You should expect to pay a premium for something out of the ordinary.

If OSH is a big box store those present up here always have very limited selections - including Lowe's (and despite their currently advertising having a giant selection - if that dazzling sheet of healthy colorful plants they show in the TV ad is to be found at an actual physical location somewhere, I want to see it). (HD has advertised having nursery experts for years, I never find any of those there either). Any established, seriously run independent nursery (that stocks Japanese maples) should automatically be a better choice. At a big box here 6 kinds at the same time would be a surprise.