There are quite a few marketplaces selling Instagram accounts but it would be very difficult to find one word username accounts over there and even if you manage the find one, they probably won't sell it cheap.
Hence, the best way is to find inactive one word accounts which aren't so active and haven't posted in a while and try to contact the owners of the account if they're willing to sell it. There are chances they won't reply since the account could be inactive for really long but again, that's the only way you can get them for the right prices.