If you're price sensitive, make sure you check eBay! That's where I got mine... a set of five Federal SS657, size 165/80R15. I read somewhere that this was the closest modern size to the original size. Many put wider tires on, but I was specifically looking for narrow, like the originals, as I really like the look on these cars. But anyway... I got the Federals just a few months ago for $50 a piece! They are really nice tires, and I picked them specifically because 1) Federal is a good name and doesn't sound like I'm putting Japanese tires on my classic British car (even though they are in fact Taiwanese), and 2) I liked the way the sidewalls look.
If you're thinking the same size, you'll find a few choices still around on eBay, from reputable sellers, for around the $50-$60 range.
I had the tires shipped straight from the seller to Hendrix Wire Wheel, while simultaneously shipping my four original wheels. The guys at Hendrix trued my wheels, installed new tubes, mounted, balanced and shaved the tires... plus I bought a fifth brand new wire wheel through them from Dayton. For all the service they did, plus the new wheel, plus the tires... I was extremely happy with the money I spent. Excellent value per dollar!
Hendrix did a really fantastic job as well. I can't believe how smooth the car drives, even at high freeway speeds.
~Robert