I have a 2009 Jaguar XF, does anyone know where they sell rear diffuser fins that fit? It's easy to find anything for later models but nothing for 2009 or 2010.

 

I dont think you can buy the fins themselves, i believe they come molded into the bumper. you need to purchase the whole bumper

 

Mina Gallery used to sell the X250 valance + diffuser as a one piece item but it seems they no longer do. It was made to mimic the later (2012 on or "facelift") XFR ribbed diffuser.
I bought one pre-painted in the correct body colour and fitted it to my old XFS, then I had the diffuser part painted gloss black.
The same item fits all X250s from 2008 to 2015 (ie "classic" and "facelift"), most times it will fit right up to the existing bumper with the locating tabs in the right places but other times you need to cut some extra slots in the bottom of the bumper. And you need both the valance and diffuser, the diffuser by itself won't work.
Lots and lots of sellers of the later X260 valance/diffuser but that doesn't come within a bull's roar of fitting any X250, and I don't think anyone sells the X250 set-up any more, well at least not as a new part.
You might get lucky and find a seller of a used part on eBay or else you can try wrecker/salvage/breaker yards. Here in Oz we have a system where most of the wreckers yards are linked by a computer database so they can search the whole country for the part you are after, dunno if you have a similar system in the US.