I am tall and thin and none of the standard sizes (S, M, L, XL) fit me well. M fits well through the body but is too short and binds at the armpits. L better, but then I am swimming in with excessive width of it.
Medium tall is perfect for me, but understandably, very difficult to find. I pretty much can’t buy this size from a brick-and-mortar store (except for LL Bean and their outlets). But I have found over the years that several clothing retailers do sell this size online. Here’s the current list of ones I know about:
eBay also has MT sizes, but many items are simple resellers of the same brands listed above. Let me know if there are others I missed!