BMS = Buick Motor Sports
This was the high performance division of Buick back in the day. It would have been equal to the GMPP or GM Perfromance Parts that exists today at your favorit GM dealership or online.
This is where you could puchase factory manufactured performance parts such as stage 2 parts, cranks heads and the such. This is where you were supposed to go to build your Pro Stock vehicle back in the day.
BMS no longer exists and all the items to produce the BMS parts have long been destroyed. Too bad too.
Anyway BMS stuff is getting rarer and can be expensive. If you need something that you want that was available via BMS, check your favorit vendor or vendors. More than likely you can get the same or close spec'd or better part for a reasonable.
Hope this helps.