Everyone and their dog sells universal caps for R134 systems but I think R-1234YF systems are different and I see your problem finding spares.
Have a look at this site. I did a site search "R 1234 dust caps ' and its lists H and L caps with M8 and M10 and also M9 thread etc. (you could maybe check your thread size with a nut or something)
Prices each are in pence but postage cost ,minimum quantity etc may be a problem.
https://www.autoairconparts.co.uk/catalogsearch/result/?q=R1234%20dust%20cap Its possible that anywhere that does aircon recharging will keep a few spares, (or accumulate a few they forgot to put back on
). If the one where you had your recharge are unable to help you may find a friendly one who will help you out from goodwill or for a small fee .
