What Are Some Good Ways To Get Newer Used Forever Stamps?

I'm fortunate that our local Bible Society store accepts donations of bulk stamps from associated churches, customers and other donors and then sells them in 1lb bags for $5 each. . Some people get rid of old stamp collection accumulations from deceased relatives by donating them to the Bible Society. The money raised is used to purchase Bibles and donate them to inmates in prisons. There are many recent and older stamps from around the world in these bags, many off paper but most on scraps of envelope. Fortunately some donors neatly clip off the stamps with full cancellations included but there are very few on full covers. Over the course of a year I'm able to get many of the latest US and Canadian cancelled stamps. I realize I'm paying $5/lb but look on it more as a donation that has returned much more in value and enjoyment to me. It's surprising how many of these used stamps have absolutely no cancellation marks on them...

I don't understand why someone would send mail to themselves? When they can just save that stamp in mint condition. Why cancel and null it's face value??

Because people collect used stamps as well as mint ones. Or, indeed, used stamps and not mint ones.

Contemporarily-used Forever stamps are less common and therefore more of a challenge to collect than their mint state counterparts.