I believe that today's used stamps are not as scarce as some people think. You can receive lots of used stamps if you work for an industrial or service company (claims, recycling center, nursing home/assisted living, manufacturing, etc.). Many government and utility companies are downsizing their buildings and services; they eliminate their customer service or drop off boxes. A local company may charge you a ridiculous service fee (several dollars) to pay a bill in person or at a supermarket or gas station. It is still cheaper to mail bills, which is why I use 13 stamps per year for my utilities company and Motor Vehicle Department.
When I purchase stamps I always ask for commemorative stamps and I encourage my coworkers and friends to ask for the same. Where I work the receptionist saves me stamps that come in on envelopes but we get very few with an actual stamp. when I get extra I will typically sell them on eBay and do OK with the price. I always use them when buying stamps from Linn's advertisers and on the SASE.
Just last week at work we received one envelope with the flag and fireworks and one with Elvis that was it out of probably 1000 envelopes that came in. Some weeks have none so two was OK, I am able to get some from France on occasion. Make sure you let your family know you collect stamps and they may save you their envelopes just help them out on how to save them; seen many where the stamp was cut when I got them.