I used carpet from The metal House:
The Metal House stock over 400 different Seals, Extrusions & Trims plus Camper & Van Conversion Products. All available with fast shipping.
themetalhouse.co.uk
It seemed to work pretty well. Only gripe is that it does 'fluff up' a bit in locations where it sees a lot of contact. For example, where our daughter rubs her foot against the sliding door panel when she's bored on long journeys.
I used a mixture of products including glue and carpet from The Metal House, clips from Transporter HQ and Megavanmats, poly insulation from Kiravan and sound deadening and foil backed 10mm self adhesive foam from dodo.
Initially I also used self adhesive foam from The Metal House but, unfortunately, the product was terrible. It was too stiff to form around the shapes in the body work and the adhesive 'let go' after an hour or so. Basically, it all fell off. That's when I ordered the dodo foam and, man, what a difference. A far, far, far (I can't overstate this) superior product.
To that end, I am curious if there are higher quality carpet options out there. The Metal House stuff was not particularly cheap and it was easy to work with but it's a lesser known brand. Maybe I'd go with Kiravan or another bigger brand next time (out of curiosity as much as anything).
I used the OEM panels with a mixture of clips:
Sliding door - Hidden fixing clips from Megavanmats.
Tailgate - Original fixing clips. On balance, I might use these on the sliding doors next time as I have picked up a niggly central locking issue and the hidden clips are going to be a pain when I remove the panel to investigate.
Side panels in the rear - Hidden clips for most and just two of the 'long' clips from Transporter HQ in the two locations where you are clipping to a piece of L bracket steel (you'll find these when you remove the panels.
Roof panels - Long clips from TransporterHQ with the exception of two spots where there is not enough space. Here I reused originals.
Rear seat belt covers - Long clips from TransporterHQ.
Can't stress enough the importance of creating neat holes for the clips to pass through in the metal work. Where I did not do this, the clips where a total PITA to fasten. As previously said, I sacrificed an old soldering iron to melt neat holes in the carpet.
Anyway, probably enough detail there to bore you to death. Hope some of it is useful.
Cheers.