Balance Runner

Dear Sir/Madam, A while ago I bought balancers for windows. I think they were 25" Series 751 Channel Balances. The other day at that same job one of the windows would not stay up. After inspection, I found the right side had the sharp point of balancer had damaged the actual runner and it has to be replaced. In your site I couldn't find the runners for the balancers. Where can I get the runners I need? I think she told me there's a few other windows that won't stay open so I think there may be more than that one window I refer to.