MC-Varuhuset at Norrastationsgatan is you best option for seal and parts from mama-yama. It makes it a lot easier if you know exactly what part you're looking for, cause you need to give the staff some assistance...
Custom parts is a bit trickier. IMHO there's no real good place for metric bikes/parts in Stockholm. You can find Küriakyn parts in most shops, but how fun are thoose?
If you go to The Parts Stop very close to Kista(http://www.thepartsstop.com/) you'll find a lot of custom stuff. They are mainly Hardley Ableson, but the shop keeper is a very nice bloke who'll help you get what ever you're looking for.
Then there's Raceway Repair(08-84 01 75)at Södermalm. Neither of these shops does streetfighter style. If you're going for that style, then internet is your best option. Try http://p201.ezboard.com/bvmaxchat . Tons of stuff on For Sale board plus the bonus of a decent lingo