Unless you know the manufacturer and can contact them directly I think you will struggle to find an exact match, but if you google plastic upvc trim suppliers you will find there are lots of firms selling various trims in short lengths, one of which you may be able to adapt to fit.

Somebody once told me that an extrusion die was quite cheap to make as it's basically a plate with a slot in it. So I got to thinking that perhaps somewhere out there you might be able to get custom extrusions made cheaply?  Google found...

http://www.3axis.us/plastic-extrusion-services.asp

"We will fabricate a minimum of 12 feet to your specifications."

Sounds good but I think it's going to be too expensive for you. The die might still be a few hundred £ and they might have to buy a minimum quantity of the raw uPVC if they don't have it in stock. Could be other set up charges? They may also only be interested if they think you are likely to order bigger volumes later?

Getting exactly the right shade of white may also be an issue? They won't be able to match a sample without a lot of work or luck.

If you do give them a call do let us know what they say!

Edit: Just noticed they are in the USA !