I've just googled for plastic rod, its not finding a source that's a problem, its narrowing down what you need from the vast array of possibles. Certainly in the UK there is one company offering white acetal rod in 10mm. I have looked at this too and think that some machining would need to be done to the rod, to make the original tuner shaft fit into the new roller. The original rollers may not be exactly 10mm. The plastic company that I dealt with in the UK, had a helpful sales staff who were willing to offer advice on the suitability of various plastics.. I don't know much about Acetal and have guessed that ABS might be better. PVC and Nylon seem to be a bit too soft. Frank Ford has used bone (machined).
"When I was younger, I could remember almost everything, whether it happened or not." Mark Twain