Well, I have a pack and a half of shingles at y house....the pack is brand new...
But on a serious note.....if you have aHabitat for Humanity/HabitatReStore nearby check there. They sell inexpensive new and used building material. Check CraigsList, FreeCycle, local paper. If that is not an option, check local HomeDepot, Lowe's etc for returned, opened/damaged shingles. If you don't want to use shingles, Tinroof or corrugated PVC is another option. I don't know about the price of tin but I bought corrugated PVC for (I think...) approx $17 a sheet 4x8
i like tin roof and pvc for roofing or even use the pvc for sides because i think it transfers heat through the roof a little better because it heats it but in the summer it stays cool. also, the pvc lets light through if you need it to. i made a couple skylights on my pen woth tan pvc