We don’t officially support AL- or NL-only leagues at this time. This kind of filter would be more confusing than anything, and if we do undertake AL- or NL-only options, this filter would just not be necessary, as search wouldn’t bring you players from the other league.
If you want to open a wishlist item for NL- or AL-only that’d be great, because there’s always one or two people who ask for this kind of support, but I haven’t seen any evidence that there is huge interest from Ottoneu users. I’d be happy to be proven wrong on this!