Resistance is resistance, any weight will work. Example: stuff one's old old gaming magazine collection +/- a history book into a pillow case and use it for squats, floor press, pushups, OH press, etc. The pillow case may only weight 15-25Lbs but its better than nothing.
Calisthenics can be made progressively harder by altering the angle, leverage, weight, range of motion, etc. Example: Pushups with your feet raised, one arm pushups, etc. Pullups are a superb exercise, if you have a basement, water heater, sturdy door, etc, you can do them from those.
http://www.dragondoor.com/articler/mode3/499/ That said, weights are the best option, even if you have to build your own weight set, find a way. I don't see anything wrong with using a heavy rock for resistance, logs, climbing the side of a small cliff, commando pullups from a tree branch, etc. Craigslist is your friend, good deals show up often.