Although we are a manufacturing shop, I get a few questions a week if someone can roll by and have us make modifications to their frame or repair a crack. My usual answer is for the person to call local bike shops to see if they have any referrals. I don’t give recommendation because I don’t want to cause any awkward/warranty/insurance situations for anyone.
What does everyone else do?
There are some builders that do repairs but but would be great to get a full list.
I’ve been asked before and did a few local repairs but have decided it’s not what I want to be doing and always takes more time than I expect. So I send them to Bilenky Cycle Works, they have a page on their website with common repair and modification costs.
I am the person all the locals refer folks to for repairs and modifications 
I have a hard time saying “no” even though it rarely makes financial sense. I just hate seeing bikes in the trash!
Since I’m still starting up, I’ve been able to hold on to repairs until I have a lull and use them to fill that time. I do find that I learn a lot for every repair I do, but it’s still a ton of NRE each time.
In the SF Bay Area, Erik Billings is the go-to guy for mods and repairs.
https://www.billingscycleworks.com/services
-Jim G
Frame repairs are a great way to see how bikes break and all the corners that are cut on production frames.
It’s important to set clear boundaries with the clients about how much time it takes to fix! @liberationfab I really recommend having a shop rate you can quote. Then it’s easy for them to say if they are willing to pay to fix it.
Andy Newlands has some great commentary about how much shop rate needs to be to avoid poverty.
Hahn Rossman