Tips on getting in with a mechanic for scrap parts

On a different note, is there any particular reason you're focusing on mechanic shops? This is usually the first place someone new to scrapping thinks of. I know I did. As time goes by, you'll get more creative in finding places people wouldn't normally think of that have a steady supply of metal. For example, I got started scrapping when the print shop I worked at started slowing down and eventually closed. Paper was their money scrap. Along with the aluminum plates. But anything else went in the trash. Everything from banding wire, empty ink cans, broken machine parts, etc. I asked my manager and he said if it's already in the dumpster, it's mine. So you don't always have to look for places that mainly produce metal waste. Sometimes it's better to find a place that the metal is an afterthought because people pay for garbage removal and you taking it for free saves them money.

on that note, there are also several garage door companies in a metropolitan area that do not have any agreements or consistent plan for their metal waste other than trash. I frequently receive a healthy supply of old garage door motors and solid extruded aluminum railings. Even whole doors from time to time.
Use commercial kitchen equipment warehouses also generate a TON of waste stainless steel, aluminum and sometimes copper cookware. I challenge anyone to go visit their premier pre-owned commercial kitchen equipment warehouse and see the sheer volume of thins they have. Some of them have agreements, some of them don't. It is a cycle, one hauler does not necessarily stay as their man forever. People move, find new careers etc etc. There is always openings into these places.

Another common business with more than enough availability is glass window and door installers. I have found that thrift shops are underutilized by scrappers and rarely have a problem jus stopping by to ask if they need to get rid of stuff. The most common things they toss though is furniture with little metal value. There are other treasures to be found as well though.
If you are coastal, the people who do storm shutters often do not have a regular scrap guy.
Solar panel installers frequently dispose of metal support frames and lead batteries!
Aquarium shops occasionally produce dead motors and ballasts, as well as PVP pipe if you like to use for your own projects.
Pawn shops frequently toss out outdated E-Waste. They are usually open to making deals if you can be consistent.

There are an innumerable number of business sources for scrap, and I can't imagine I have made deals with even a fraction of them on top of what I have listed. Diesel mechanic shops for heavy equipment tend to go unnoticed sometimes.
Sometimes smaller independent retailers have a cluster **** of old fixtures in a back room. Asking frequently can generate something about 20% of the time.

Everything in our lives requires metal. Pool supply places generate bent and damaged aluminum tubes, ladders, chairs and occasionally motors. Just imagine everything in life you enjoy, and there is likely a business that generates scrap related to it that is waiting to be served.