Used tricycles can be purchased on eBay or Amazon.com. Both these websites carry children's tricycles, as well as full-sized tricycles for adults. Prices range from under $100 to over $1000, depending on the quality of the product, as of April 2015.
Amazon.com carries a few adult tricycles. Some are upright models with a basket behind the seat, between the back wheels. Others are low-riding models with a reclined seat close to the ground, while the pedals are attached to the front wheel. EBay carries similar models, as well as some spare parts or accessories. Both sites have used tricycles for children.