My local paint supplier will put any colour you like into a spray can. Just call up some bodyshop suppliers close to you to see if they provide the same service.
You'll probably be given solvent basecoat, which is matte anyway, but it should really be coated with a matte lacquer. Then again, you say it doesn't have to be a great job, so it's up to you whether you clearcoat it or not.
Do you know the colour code of the purple you're after? If not, go to your local bodyshop and ask to see a selection of their colour chips (also called swatches or spray-outs). If you can convince them that you'll give them the respray to do they are much more likely to oblige.
One more thing: do you know anyone with a compressor and spraygun? It might be worth asking around, your paint will be much, much cheaper if you buy it as a premixed litre to spray through a gun, rather than in aerosols.