I recently moved to an apartment in San Francisco with a very small footprint — its saving grace is that it is connected to a large shared yard, a huge boon for an aspiring gardener in an urban area. I've been given the go ahead from the landlord to plant, but find myself reluctant to spend too much on something I can't take with me.
While I regularly check Craigslist for free and inexpensive plants, it's the luck of the draw. Determined, I started browsing a few garden forums. Many of the bigger ones have entire portions dedicated to exchanging plants, cuttings and seeds. You can put out requests for specific plants and offer up what you have in excess. I love the idea of a community centered around a shared passion that enables its members to improve their environments for little to no money.
There are also organized plant exchanges that take place in person in many major cities. San Francisco has the Annual Lakeshore plant exchange, as well as smaller neighborhood-organized ones.
If you're interested in getting involved online, Folia and GardenWeb are both great places to start.