What he said. Dallee 367. Link has easy to follow wiring instructions.
There are other suppliers of flasher modules as well - seem to be in the $20-$30 range. If you want to dig deeper, search OGR for something like "154 flasher" and you will find dozens of ideas from DIY circuits (requires soldering) for a couple bucks to more complex (expensive) flashers such as with accompanying bell sound.