To my knowledge, most accessories (but not all) beyond performance mods and knickknacks (those which would also apply to the Grom and Monkey), folks are ordering from Beezdeals.com (they're in Thailand, where your bike was actually mfr'd.), reputable eBay sellers (btw Beezdeals also has an eBay store, and it can be cheaper than buying from them direct b/c of eBay's sellers shipping rate association), Webike Japan (they're simply an extended distributor who reps grundles of mfrs.), some Amazon occasionally, and also other Thai indy trinket sellers, depending on the item.
For racks and headlight guards, most options seem to be either eBay (e.g. Beez, et.al.), Beez direct, Webike Japan; not much IME stateside (forum long ball hitters, please chime in here and correct me IIW).
Your oil cap is identical to the Monkey's, so there may be options available from Steady Garage (CA), Man In The Box (mnnthbx.com, TN), Hard Racing (NC), DrowSports (CA), to name a few -- not sure -- check 'em out. If you want a metal oil cap with an ape emblem engraved on it, I know they're available and they'll fit the Donkey (it's no coincidence the monikers are just one letter off!).
Dime City Cycle in FL (cool indy parts/custom shop) has bent over backwards to help me on Monkey and W650 stuff (analogue tach, various kabobs...), so take a look there even if it's for fun -- may be a long shot..
There's a cool CT125 temperature gauge oil cap on eBay (one seller for e.g.:
https://www.ebay.com/itm/373303722786?var=642263909820 ) -- have one by the same mfr. on a W650 and it's worked well for years.
You might know... it's totally safe to order from Beez, eBay, and Webike Japan, although recently on an order Webike hasn't actually taken good care of me regarding some bad parts and mfg. mistakes from one of their mfrs. They're very polite and apologetic, but slow, ineffective and failed to make it right. That's not to say they're slow to deliver purchases, they're actually quite fast. Delivery from Thailand and Japan are also faster than you might think.
Good luck man!