I use a cheap one from China, that does have n-key rollover, you can probably look up on the web somewhere which ones will definitely work.

Mine looks like this:

https://www.ebay.de/itm/312708601103?epid=1219753984

P:S: According to this post you cannot get true n-key-rollover via usb connection no matter if you use an adapter or not. You will only be able to press 6-8 keys at the same time. It seems to be good enough for me so I never noticed. If you are a purist, your best bet would be installing a pci card with ps2 connector I think.

https://tl.net/forum/tech-support/170648-ps-2-usb-adapter-nkey-rollover-q

https://www.daskeyboard.com/blog/das-keyboards-ps2-pci-card-recommendation-for-n-key-rollover/