Not too much selection in ultra wide street tires for 15 inch, nor 16 for that matter.
I found a NOS set of Jongbloed built BRM style pin drives for my GT40. Fronts were 16 x 9.5, rears were 16 x 12. Due to the lack of tires for the rears, I ended up having Jongbloed spin new 17" rims for the rears. Left the fronts at 16". Michelin, Yokohama, Pirelli, Bridgestone, etc. all make suitable tires to fit 16x9.5's and 17x12's.
The nice thing about the Jongbloed wheels is their modular 3 piece design. Need a larger diameter, or different width or offset, just get new new rims spun. I think Jongbloed gets $800-$900 per wheel (Magnesium centers, aluminum rims).
It cost me just over $400 per wheel for the new rims.
http://www.jongbloedwheels.com/jong_vintage.html
Dave