Welcome from the UK, GrandpaDave!
I don't collect PM trains, but I love trying to track down missing PM parts. You are right that the Replacement Parts service (aka Direct Service or DS) does not offer parts from sets older than about 8 years. Which leaves eBay or fellow collectors to supply missing parts much of the time. Many of us hope and pray that older parts will be re-released (is that a word?) in current sets, so we can buy them from DS.
Show us the specific parts you want, with part number if possible (and a link to the part on PlaymoDB if it's shown there) and someone may be able to help!