I would go to the parts office of a dealer and speak to them. A full sized spare can be bought, but don't forget, if you want to store it under the car you will need the support cage and of course you'll need a jack and a wheelbrace too. Maybe.
Don't forget though that spares stored under cars are liable to theft. This happened to me on my last Doblo; they used wire cutters to cut the cage and nicked the wheel in the night: bastards! So I bought a new steel wheel, had a cheap tyre fitted, and kept it in the boot. I am doing the same with my 'new' Doblo. Yes, it takes up space, but they can't nick it! And yes, with ESC etc. you should have four identical tyres on the car, but I intend to use the spare only as a get-me-to-the-tyre-shop standby anyway.
I don't have a jack or wheelbrace now by the way, but when I had a puncture at 03:00hrs the AA used their own kit, so there was no need. I have no intention of ever changing a wheel myself again anyway
I hope this helps. Now I have the useless gunk thing the car came with and a spare wheel too, but no jack etc..