Could you make them out of tent pegs??
I dont know if theyd fit in the holes. I suppose they might be a bit long and stick out, which could be dangerous.
Just an idea though!
I think you'll struggle as most places seem to just do the safety cups these days. I'd try contacting a show jump maker like these people http://www.west-country-show-jumps.co.uk/horse-jump-accessories.asp and ask if they can supply just the pins. BTW I know nothing about those makers of if they are any good - they were just the first ones to come up on a google search who didn't supply polyjumps!
Find a local metal fabricator and go along and have a chat. Take in one of the ones you have to copy. They could probably make some for you if they are fairly simple. If they cant do it they may know someone who can. Wont necessarily be cheap though as you will be paying British fabrication prices! My husband does this for a living and he cant make jump cups for the price you can buy them. Although if he did they would be superior quality!