11BLAC Mainspring

I've got a Sea King N0 and I'd like to replace the mainspring while I attempt to service it (I'm new at this). The watch does run but it's about 3 min slow per day according to a timegrapher and the amplitud is only about 170-180, so I figured I'd swap the mainspring out and see if that helps. Where can I get one? Searching on this (e.g., Otto Frei) yields no results. There are many other Bulova mainsprings on that site, but not 11BLAC. Is there one that's equivalent?