Any stores which sell car/van stereos should be able to help. If not a little vandal I know told me to put the stereo in the freezer for 10 minutes and it's decoded. lol Not that I'm encouraging thieving.
The freezer thing is an urban myth - and does nothing except damage the radio.
Ask the people who you bought it from if they have a code
The original supplying dealer may have a record of it