Welcome! ThayerRags.com is one source of parts for older machines. He's also a member here though he doesn't advertise here. It's against the rules.
A good source for common replacement items is Sew-Classic.com. The owner is very knowledgeable and they have a good reputation here.
jihmnj is a member here with an Ebay store. He sells obsolete sewing machine needles among other things. Your National uses a 20x1 needle.
http://stores.ebay.com/Jacks-SM-Repair
Also check the sticky on vintage machine resources.
Hope this helps,
Rodney