1) Go through the "Parts for vintage synthesizers" sticky-thread at the top of the "Help!" forum, and visit the websites, call, or email the sources you see listed there - many of them are worth at least trying, and many of them have parts in stock that aren't listed on their websites
2) post a "want to buy" (WTB) ad in the for sale / wanted section listing the parts you're looking for in case someone has them for sale
3) get in touch with a vintage synth parts/service place, and try to find copies of old service manuals and parts lists, and try to find out what parts are "in common" between your SH-1 and other Roland models - for instance, the bender joystick-wheel and slider 'knobs' on your SH-1 look a lot like those on my SH-9 - are they the same parts, or are they incompatible? I don't know.
4) Armed with this information, also check eBay regularly - parts salvaged from other broken synths, and new reproductions of parts (particularly for Roland) come up regularly, but are not always up there, so you sometimes have to keep checking again and again.
5) Unless you own a really "iconic" Roland piece (808, 909, Juno, etc.) there may not be "new reproduction" parts available anymore, so you're probably looking for "salvage" parts "cannibalized" off a dead / scrapped unit - this can take time, and sometimes you just have to make a mental "wish list" of "original replacement" parts you're looking for and keep remembering to check your list of places every so often. For example, I found replacement keys for one of my synths immediately, but I'm still trying to find someone willing to part with a few replacement slider-knobs for it. Having a few "in the process of trying to track down parts for restoration" keyboards lying around means ya always have something to do.