Any 12 V source would be OK. I have used a UPS battery before. Those 27A batteries are very inexpensive when ordered through eBay, so I like to use them because they are very small and handy to put on a breadboard. A car battery may be OK, but measure the voltage. If it has 13 or 14 volts that may be too high. Since those Tiny85 boards are very cheap, you would not be risking very much by experimenting.