Here is a list of tires that are available in the US, in 3.25-19 and 4.00-18, the stock sizes for many vintage bikes:
• Bridgestone Battlax BT46
• Metzeler Lasertec
• Michelin Road Classic
• Michelin Pilot Activ
• Avon AM26 Roadrider
• Metzeler Sportec Klassic
• Avon Roadrider MKII
Of these, I found that the Bridgestones are usually the lowest priced. A few weeks ago I bought a set from Chaparral for $114 - front and $140 - rear, with free shipping.