Teac Canada is usually pretty good at keeping heads in stock and I would assume TASCAM America does as well.
If they are sold out, JRF magnetics manufactures heads that will work with the unit and actually exceed the original specs of the factory heads so, no worries!
You are probanly better off trolling ebay for old, defective but not frequently used A-3340's. I got one for just $30 and the heads were in great shape.