I've used loads over the years (one of the very first testers of printed deal papers back when I worked at a computer store and got friendly with the woman who sold us our printer paper). The one's I'm most fond of using are those from Craftycomputerpaper.co.uk for the UK and I occasionally use the ones CPC (cpc.co.uk) sells, but the CCP ones are of a better quality and very reasonable.

One note would be to read up on home-printing decals, because the commercial ones will be of a higher quality, and if you do print home-mades, convert bitmaps to vector graphics so you don't get skanky edges.

Here's a mini review of sites:

www.mksmc.co.uk/hintsandtips/decals.htm

Other good suppliers in no particular order or country (these are those that I've tried and liked, some may require importing, so if you just want a try, use CCP as above):
www.mcgpaper.com/decals.html
www.hyaz.com/
www.beldecal.com/model_decals.cfm
www.decalpaper.com/
Regards - Elliott