In the spring and summer, Starbuck's has their Grounds for Growth, where they bag and give away free used coffee grounds. Spread these around under roses and any other acid loving plants. Recycling at its best!
One can probably find coffee grounds at several different places close to home--a nursing home, a hospital, a convenience store, a small cafe, a nearby restaurant. If you talk to the manager they may be willing to save some for you.