I too have found raised white letter tires for our Mustangs difficult to find. Like everything else, I think it's cyclic. Raised white letters are so yesterday, until tomorrow, when someone will put a set on. Then someone else will think they're cool, and the whole fad will start again. I personally like them, but then I grew up with "Polyglass Bias-Ply" tires on my '68 Mustang.
But I find it hard to believe tire manufacturers have given up the free neon ad space, especially when everything made today is little more than a commercial.
How true, it's only uncool until enough people want them again!