15X 5.5" I believe, fairly easy to find in the U.S., as well as inexpensive. Are they any good? I have no idea. They're not very popular right now, AFAIK, but I think they are a decent looking wheel.
True--there were only six wheels that I can think of that were factory options on the four-cylinder 914s.
Early cars had 4.5" wide steel wheels "standard", to keep the base price down. Very few cars in the US, at least, kept their 4.5s--the dealers added other wheels to jack up their profit margin.
Another option for early cars were 5.5" steel wheels.
Later, the 5.5" "styled steel" wheels supplanted the earlier design.
Early cars (at least 1971, possibly 1970) could be fitted with 5.5" wide Pedrini cast aluminum wheels. A revised version was offered in 1973 that fit the hub-centering rings on the front of the 73+ cars.
The Mahle 5.5" wide cast-aluminum wheel was introduced in 1973, I believe.
Fuchs supplied a 5.5" wide forged-aluminum wheel, starting in the 73 model year as well.
All of these wheels were 15" in diameter.
All other four-bolt wheels fitted to 914s were aftermarket, though many were installed by the dealers. (See earlier comments about "profit margins".)