I would replace both side at the same time, use some PB Blaster on the bolts and let them sit over night, I would not just throw on used ones. do you know it is the caliper ? are the pads worn down all the way ? how are the rotors ? do you live in the USA ? I know you said there is no money, if the pads are worn down I would throw on a set then save up for loaded cal's and maybe rotors.

2001 5sp. K&N stock filter 215/70/15
If your check engine light is on at least you know the light still works