Look for IRC or Cheng Shin 3.00 - 18 (or 3.50-18 for rear). From an old post on the Yahoo BS forum:
Re: Replacement Tires
I recently spent dozens of hours on this same issue, and ended up
buying IRC street pattern tires for my HS, because I don't really
expect to ride around on dirt. Mine came from Parts Unlimited, but
you have to order through a dealer.
IRC-14 GS-11 Front SIZE: 3.00S-18 BLK $59.95
IRC-08 GS-11 Rear SIZE: 3.50S-18 BLK $67.95
My total cost for the tires, 4 new IRC inner tubes and shipping was
$202.00
I know for a fact that the 3.00 front and 3.50 rear will fit just
fine, although you will experience a minor aggravation getting the
front wheel to squeeze into the fender braces - let the air out
first, then reinflate after the wheel is in place.
These IRC GS-11's have an old-school appearance, certainly look OK on
the bike.
http://street.parts-unlimited.com/49/317/4095324If shopping Dennis Kirk, be sure to click the "availability"
button ... my experience is that they probably don't really have them
in stock. Same thing with Tucker Rocky ...
The Cheng Shin tires are probably most like the originals, in
appearance and performance. I know that lots of guys who run vintage
bikes use these by default, because there just aren't any
alternatives in the skinny 18's.