As common as dirt.
More Traditional Proverbs:
He does not live in vain who employs his wealth, his thought, and his speech to advance the good of othersTraditional
Every garden may have some weeds
Traditional
Many candles can be kindled from one candle without diminishing it.
Traditional
A lawyer's opinion is worth nothing unless paid for.
Traditional
Bit off more than you can chew.
Traditional
Thick as a brick.
Traditional