I've never done this, but I wonder which parts you mean. If they're fairly straightforward in section, you might be better off just taking the old parts, or a drawing, to a small local joinery company and asking them to make replacements.
We have renovated almost all the sash windows in our Victorian house. We used Reddiseals (www.reddiseals.com) for the staff and parting bead (timber, not plastic) and the brush draught-proof material, which just slots into the groove of the beads. They were very quick with delivery and the timber was well packaged in a sturdy 3m tube. Best of luck with your renovations, there's nothing like a properly working sash window - brilliant!
~p_larua