Know the dimensions of your truck bed, spend as much time looking at a lot of web sites as you can, expect to spend a long time searching for a used camper and have the cash or cashiers checks ready, then jump on the one that fits what you want and your truck. Before I knew how valuable FWCs were, I passed up a local unit that would have fit my 6 year old truck well, then waited 16 months to find one 300 miles away. During my drive there Friday after work, the seller called to tell me that another party was interested, but since I had contacted him first and I was driving a long way he wanted to give me the chance to buy it, but it would go to whomever offered him the asking price first.
By 8:00 AM Saturday, I had agreed to buy it. Just then his wife pokes her head into the garage and asks if the other guy should drive 100 miles. I gladly told her "no, we just shook hands." I had cash and a truck ready to carry the camper away.