Your biggest problem is not finding it, it is shipping it. You can find monitors from $79 but it will cost $50 - $80 to ship it (and you have a fair possibility of damage in shipping)
Your best bet is to look locally. You should have one or more used computer stores near you, auctions, Computer user group garage sales, government surplus sales. Pretty much any large screen CRT is compatible with a Mac PROVIDED it is a true multi-syncing moitor. Some older monitors and some that came with Sun and HP workstations are fixed to one sync frequency, and you may have an issue getting a mac to recognize it. Even the early single-frequency Apple monitors are not compatible with G3 and above.