+2 on Zephyrworks. I just ordered three boom-end sheaves from him. The small forward sheave for the reefing line was a 3/8 ID sheave (it rotates on a stainless 3/8" bolt). The larger, aft sheaves rotate about a press-pin in the aft-boom sheave box. I mistakenly assumed these were the standard 3/8 ID as well. The press-pin is actually about .378" diameter when measured with a micrometer.
The original sheaves I ordered did not fit, but Ed allowed me to ship them back so he could re-bore them and return them to me (at no additional charge).
So, measure your sheaves with a micrometer to get it right. Inner Diameter (ID), and width (of the hub or bearing surface) are most critical. Outer diameter and groove depth are pretty nominal. And, as Bruce said, the walls on the sheave-box were not square--I just went with the same width as the old sheaves, even though this leaves some side-to-side play in the sheave box. You can "shim" them up further with nylon washers from the hardware store, if you wish.