Here's one article to get you started. Do some googling about thrift store excess. Let go of the guilt.
I bought almost all my clothes from thrift stores while I was in grad school, and spent some time talking to the employees. Every single one said they received far more donations than they could ever sell (even the tiny local ones!), and routinely dumped massive amounts of clothing to other dealers (rags, etc.). If they physically can't move all their inventory, any purchase is better than no purchase. And I've never noticed "lack of supply" as an issue in my local thrift stores. I never feel guilty, and usually feel really good, about thrift store shopping!
Yeah they get way more than they can handle. I donate around the backside of my thrift store, and they have literally piles upon piles of stuff on their loading docks.
My wife had done some volunteer work at thrift in a poor rural area of Florida. She would literally show up before 8 AM, leave at 5 PM, bust her ass all day, along with at least one other person, and barely make a dent in the incoming supply of clothing. It was nothing to have a single donation of 6-7 stuffed 30 gallon garbage bags full of top quality clothes, some never used, with tags on. The store was an extremely successful non-profit, pumping almost a million dollars back into the local community every year. The clothing had three grades. Top grade headed straight to the racks in the store. Grade two went to local non-profits that helped the extremely poor and homeless, to be distributed for free. Grade three was picked up buy an NGO that shipped it to places like Haiti to be distributed for free.
When it comes to "hurting the poor", everywhere that we volunteer is quite the opposite. Not only do the poor, rich and the rest of us, end up shopping at the same place, for great bargains, but thrift non-profits can pump shocking amounts of cash back into the community. A $6 tee shirt from Walmart might return a few cents into the local economy. A donated clothing article, that sells for $4 at a thrift non-profit can put most of that cash right back into the community. I would rather see my money end up going to my local Habitat for Humanity project, or homeless outreach, that ending up in the hands of the Walton family ,as they stockpile their billions.