I prefer to sell Renold chain. Excellent quality, good fit and not as wide on the outside as many other makes of chain so it doesn't rub against the chaincases.
How long? Please count the rivets on the side and don't include the connecting spring link.
Even lengths are cheaper.
To make an uneven length, say from 100 to 99; cut of to 98 and add a 1 1/2 link. This must be riveted, so you need a rivet link as well.
"I want to fit an Oring chain?!" Good luck. Oring chain is too wide on the outside and will kill your primary chaincase. The same goes for most modern motorcycle chain. The stuff is too thick on the outside.