My first foray into used maternity wear was a happy accident. I was at my local GCF store, thumbing through the racks, when I came across several shirts and sweaters. Most of them had hardly been worn, and for only $3 each how could I not buy them all? If you are looking for a specific item this isn't the way to go, but it's great for finding random separates. I have found it is very easy to build a wardrobe with very few pants and skirts as long as I have a variety of shirts to change up the look.
Finding clothes on the cheap is good, but what would you say to getting clothes for free? Yes, free. If you haven't tried it I highly recommend you look into Freecycle. Freecycle is a network for people to give and receive used items within their community. I won't go into the details of how it works, but people can advertise an item they wish to give away or people can ask for an item they would like to receive. Several weeks ago I requested some winter maternity items, not wanting to spend too much money for things I would only be wearing for a few weeks. I got a response right away and a very nice woman gave me 2 pairs of pants and 4 tops. Also, people are often giving away baby items so this is a great resource for things like toys, books, games and gear (strollers, high-chairs, swings etc). This is a great organization - by connecting people that have things with people that can give them a second life it greatly reduces the strain on local landfills.
Additionally, Craigslist and yard sales are great places to find used baby goods on the cheap. With warm weather ramping up I plan on hitting the yard sales in the next few weeks. I'll be sure to show you what I find!