No idea at all; I'm not even sure if shops offer this service. I suspect that my local shop might offer to do it for me for free, since I buy a lot of stuff from them and send other people to them - but they may not, if I didn't buy the pack there. Also, you'd need to get comfortable with the staff's expertise; their younger salesmen may never have seen stays in a pack, depending on the brands they sell.

I did just think of another possibility - if you have a college nearby, contact their outdoor program (most have one) and you may find some of their staff willing and able to help.

Also, if you've got access to a copy of Colin Fletcher's Complete Walker series, I seem to remember that he discussed bending stays. I'm not sure it's in Complete Walker IV, or an earlier edition. I'll try to remember to look tonight, and let you know if I find anything.