Do you still have have the original?
- Watch the parts & accessories in the EE, antique & rimfire sections
- I've found obscure parts for antique winchesters on eBay, from time to time
- check Numrich gun parts, they ship parts like sears to Canada (part value has to be under $100, I'd be very surprised if it did)
- obviously exhaust Google as much as possible
As a last resort and if you have the original as a template, bring it to a gunsmith and see if he can build you one OR repair the original (build it up and file it back). If you're good and confident at metalwork and the part is uncomplicated, you could consider building one yourself - but I'm not going to recommend you make something like that just because if a part like that ever fails on you, the consequences could be real bad (just my disclaimer).
Hope those sources help you out