Akihabarashop.jp or Gremlin Solutions in the UK.
Seimitsu parts just aren’t as popular as Sanwa in the US which is a darn shame.
Part of it has to do with the people in tournaments. Most of them use ONLY Sanwa parts and because those are the only parts in most mass-market joysticks that’s all they’re familiar with. Most people DO NOT swap and experiment with alternate parts because of the expense. Stick modding/customization is not a cheap hobby when even an individual buttons costs on average $3-$4.
(Happ parts are cheaper but are generally frowned upon because they’re not “authentic Japanese” parts and aren’t used in the latest Asian arcade games. I just never cared for the feel of those Happ buttons and levers in the American arcades, period. Very uncomfortable for my “child-like” hands. lol Made switching to the Sanwa buttons and Seimitsu joysticks easy! There was no lost love for Happ from me…)
You’re limited to transparent Seimitsu buttons a lot of the time because Seimitsu had them first, people want the art and LED customizations possible with them, and they’re still cheaper (and somewhat easier to get) than the newer transparent Sanwa buttons --, but as far as joysticks are concerned it’s usually only LS-32 and LS-56 series (with the LS-58 looking more and more like it’ll replace LS-56 inventory entirely at a lot of shops). The joystick situation is as a big a deal to me because I like the LS-40 about as much as the LS-32. Way, way harder to find and buy that from a US videogame shop. The shops are going to stock what they know will sell for sure.
IF you have to import from Akihabarashop.jp, it’s generally better to place a bigger parts order because of the shipping costs. (Go with Aki if you know you need at least $60 worth of parts that may include that limited edition glow-in-the-dark transparent glittery balltop handle. People have paid $20 just for an exclusive glitter balltop from Aki. I kid not! What’do I know though. I have around a half-dozen colors of meshball handles!) If you don’t do insured EMS you’re taking a very big risk that your order will be stolen/lost somewhere along the way. (Happened to me, but I had a happy ending to my story…) They still have the most diverse selection of Japanese arcade parts and are generally the only place where you can buy individual pieces for practically any Japanese hardware. Most places will sell you a whole joystick as a replacement but very few sell the PCB’s and springs for practically every joystick as individual pieces. The same goes for button parts…