Any of these would work as Libra Symbols :p
Zwoti already posted the actual universal symbols that I would have posted
The scales also represent Libra, as interpretive symbols
For tattoos, some Libras combine the symbols, and attach scales to the bottom of the other symbol:
There's also the Libra tribal design:
Also consider colors...I have no idea why Libras love bright (usually shiny and glittery) colors so much...Especially pink and purple...And what's with the butterflies?