I can't tell your location, but according to CMSNL.com the only 1991 model was Switzerland?
Anyhow all 4 signals have different part numbers. All of them fit all years NX250 and the CB500 from the mid 90's that we never got in the States either.
https://www.cmsnl.com/honda-nx250-dominator-1991-m-switzerlandmk_model3843/partslist/F__2400.html#.YjX6pmkpCyVI advise against using used plastic turn signals, their rubber stems and plastic housings didn't hold up very well out in the UV light and most of them are dangling by their wires or they're held together with electrical tape and a zip tie to keep the tape from unwinding...
If you want new OEM they're available but will be pricey. The shape of your bike and how you'll be riding it should dictate what you'll use. If it's a pristine Survivior I'd put on OEM for maximum value. If it gets ridden like Honda intended I'd cheap out and get a set of 4 aftermarket. On eBay I find sets of 4 down as low as ~$40 with free shipping, sold from china, Taiwan, the UK, and the US, so lots of choices.
Another is to check out your local Craigslist motorcycle parts section for parts that might do the job at deep discounts. Lots of us have parts we don't want to throw out, but won't ever use. We'll never get rich selling them but losing money is easier knowing that parts will get someone else out riding again.
https://www.ebay.com/itm/173332617291?hash=item285b6dd84b:g:ZyUAAOSwm4NbBiQy