Craigslist is a decent place---but it really helps to know exactly what you're looking for or you can end up with the reefing equivalent of bell bottoms and love beads. IE, a LOT has come down the pike even in the last few years.
SHop visually online and look at features. For a reef you want 'drilled, sump, overflow, and as large as you can manage. This is a hobby in which 100 gallons is considered the low end of large, and 50 is called small. 30-40, however is commonly offered online and in shops.
You need to choose between acrylic (light but scratches easily) and glass (really heavy, especially at greater thickness. And optical quality varies.)
Also, talk to your lfs. Many if not most will happily order you a tank of the size you want, regardless of what they have on the floor. For a 'fish-only' tank, size is often a must. But a successful coral reef can be down in the nano-tank range, with just a few very tiny fishes.