h40.5 bow roller source - Excellent question!
I also needed one. I searched everywhere I could think of. I did find some offerings well over
Fifty Bucks 
, but being
cheap 
, I refused to accept that.
It is easy to find the correct size roller for a trailer for $5-6 (that's more like it) but it has a 1/2" hole and not a 3/4" hole.
Well I tried drilling one out to 3/4"- this was a failed plan. There is no good way to hold the roller while drilling with a drill press and you still need a non-standard drill length. And even if it did work, I would have to forever drill every replacement.
So, what's the answer?
I took myself down to my local hardware store and bought a 4.5" stainless cap screw with a couple flats and a locking nut. Then I bought two shouldered bushings made of bronze that were 1/2" ID and 3/4" OD from Tractor Supply Co. These bushings were too long, so I cut them down in my lathe (everyone has one of these in their basement,right?) to have a shoulder length of 1/8". The thickness of the stainless anchor roller is 3/16", so the bushing fits in nicely and converts the 3/4" hole to a 1/2" hole. Now I can just buy standard rollers when they wear out and change them in less than 5 minutes.
See the pictures below for the installed result. Sorry I didn't think to take pictures of the bushings and the machining steps. I can machine these bushings for anyone who has a need, but I will be off sailing for a while.