You can actually make your own by buying the paper itself. It comes in two different forms, clear and white. White is the best as your printer cannot print that colour. Also you need the type that matches your printer, inkjet or laser. If you use a inkjet you need to varnish the decal to seal the ink before you apply water to it before fixing it to whatever you are applying it to.
To date I have not done it myself as it is a long term project to do so only have researched into it at the moment.
Ebay usually has the paper up for sale and there is a few websites around on how to do it.