I ordered a used National from Elderly and felt like I got a good deal.
Artisan Guitars in Nashville usually has a lot of Nationals in stock.
You can order a National through certain artist-reps including Catfish Keith and Bob Brozman. I custom ordered a National from Catfish Keith. It took a while.
Twelfth Fret in Toronto. Not only do they get some really nice ones, they actually will set them up properly based on your preferred tuning and whether or not you play mostly slide. Can't recommend them enough.
Lark Street Music in Teaneck NJ is a solid National Dealer, always has great stuff in stock and more inbound. Might be near enough to you to stop by too.