Seeks parts for antique wood venetian blinds
I know this route might be more expensive - but that seems like such a simple part to make. Any cabinet shop or furniture repair can do that.

If I were to do this myself with out a lathe - I would buy a wooden dowel rod from Big Box Hardware. Cut it down to about 3 inches. Drill out the center, and also countersink the center. Then I would use sand paper and a file to shape my own matching roller. Stain and be done with it. 

Tedious yes sure - but I would have my roller.

If you have a small drill press all the better - because you could put the dowel in the Drill press and use a dremel tool to cut and sand as you have need and make it lickity split. I would say you should be done in about 30 minutes or so..

Below are not endorsements of products or services but provided as useful information..

There is also Habitat for Humanity which might have some vintage stuff - or antique shops. 
Especially considering this:

Wooden Thread Spools 

Or 

Order custom made wooden roller.
Link to above in the open: 
https://www.etsy.com/listing/258307687/wooden-roller?ga_order=most_relevant&amp;ga_search_type=all&amp;ga_view_type=gallery&amp;ga_search_query=wooden%20rollers&amp;ref=sr_gallery-1-16