I've often bought rims from MikesXS, if you happen to find the matching spoke count etc. You need to consider spoke count and the hub diameter. A rim made for a wheel with a large drum brake hub has different spoke angles than one made for a smaller diameter hub carrying a brake rotor.
For example - 19" 36 hole rim which I imagine would fit the front wheel just fine on your RD350 - appear to be out of stock, but you get the idea of what's out there. They have them in-stock in black for $48.
https://www.mikesxs.net/yamaha-xs650-h-1-85-x-19-x-36-hole-shouldered-rim-aluminum-oem-94418-19145-00-for-reference-only.html
https://www.mikesxs.net/yamaha-xs65...ck-oem-94418-19145-00-for-reference-only.html
They don't seem to have 18" in the 'shouldered' style, but they do have a variety of sizes in the more traditional, original looking 'WM' profile albeit for more money.
https://www.mikesxs.net/wheel-rim-wm-multiple-sizes-36-spoke-aluminum-universal-silver.html
For spokes, I've always bought from Buchanan's. Stainless spokes and nipples are around $100/wheel and they will custom make them specific to the rim and hub combination. They just need to know the rim diameter (19, 18 etc.) and the hub you're using to determine the right length and angle of the spokes.
Then I've usually paid $50/wheel to have them laced by a local guy who did a bunch of work for the vintage racers in the area. Your local bicycle shop might be able to do them or you can tackle it yourself (I'd pay personally).
So for about $140 (rim) + $100 (spokes) + $50 (lacing) per wheel you can have effectively new wheels. Replace the wheel bearings while you're at it.