Hi Micool,
I don't know of anyone who supplies pretrimmed door cards, but I'm sure there are suppliers out there.
I have always made my own. They are pretty simple. Take the old one off, use it as a template on some 3mm MDF from Bunnings or where ever, cut it out, swap the clips over and done. You can cover it with all sorts of different stuff, vinyl, cloth, foam backed fabrics, just head down the local fabric store and poke around, you could even just paint them if you sealed the MDF first. I've even used marine carpet to do the same for some door cards in my 4WD. Contact adhesive in a can will stick anything well enough to the MDF. This process is also used for the rear cards as well, exactly the same. I've often thought about using the original door cards as a template for sheet aluminium for that "Race Car" look but haven't done it yet.
Here is a bad picture of my door cards I finished late last year. I used 3M 1080 Dinoc "Carbon Fibre" vinyl wrap to cover them. The vinyl wrap is very thin and it shows up every imperfection. It's hard to see in this photo, but IRL you can see the clip heads etc. All up to make these from scratch cost about $20ea i suppose, I wasn't really counting.
http://users.tpg.com.au/wasabipn/Mini/20130420_140502.jpgThe best thing about owning a Mini is modifying them to suit YOUR taste. I'm sure some people think my door cards are stupid or ugly, but I like them and that's all that counts

Doing your own door cards is an easy way to get your Mini looking how you want it without being resrticted by what is commercially available. And it's cheap enough to do again if you change your mind or style.
Hope this helps man!
Cheers,
Keels.