@trendless I would start by just nicely asking an employee if they could start accepting them or a manager if you can. Then if that doesn't work it really depends on what kinda thift store it is.
If its a locally owned reuse type of store offer to volunteer to refurbish computers from them to sell in return for some of the stock that comes in.
If it is a corpo store, its gonna be hard. I'd say best bet is figure out some way too flood their google reviews with needs used computers instock