Prewound bobbins for Brother 8500

Okay this may be a stupid question, but what size of prewound bobbins should I buy with Brother 8500? I’ve always used my machine to wound bobbins, however I’ve heard better embroidery quality is gained by using prewound bobbins. I know it’s in the manual but I can’t find it! PLEASE anyone out there with the same machine model tell me what size I need to buy. Thanks a million.

Lynn