How to get the best $ deal on replacing rear brake pads for a Subaru Impreza, & do I buy the parts myself or does the mechanic do that?

Rear brakes aren't that hard to do.

1 if they are Disc brakes meaning they have rotors n pads it's super easy to do.  YouTube it and do it yourself, if you have common tools.

2 if they are Drum brakes, meaning they are enclosed inside a dumb I wouldn't replace them.  Remove the outer drum and spray them with brake cleaner.  I have never had to replace Drum brakes before.  They will get noisy then I'll clean them.

3. Not all but most shops will charge you List cost for parts they get at wholesale price.  If you can get the parts yourself at a wholesale cost awesome.  But depending on who you know or what accounts you deal with, getting wholesale pricing can be a pain.

Best of luck.

I normally get mine done when serviced as they dismantle and clean the brakes anyway, they don’t charge extra for fitting new pads. However, they do of course cost full retail , about £100 in UK.

Check Ebay or Amazon for the parts.

Mechanics don't like people who don't know what they are doing buying parts.

I only use premium brake pads and rotors when doing brake jobs. Advance Auto sells great Platinum series brake parts.

You want to go on eBay and see what you can get the pads for yourself. I took a car see the mechanic and they wanted $300 for the fuel injectors plus the money to put them on. When I found out the OEM fuel injectors we're only $50 for the whole set of 6 on eBay I was really angry what kind of a rip off are mechanics trying to pull on people charging you $300 for a part that only cost $50 please.

You should be able to do them yourself  if they are disk. Any auto parts place can get them for you, there are also many places on line you can order them from. Rear brakes can get tricky from removing the drum to proper adjustments. If you have a mechanical understanding, then there are enough videos and crap out there so you can figure it out.

You can buy the parts yourself, denying the mechanic the profit so they'll charge you more for installation.  And,  garages will seldom guarantee the job when customer supplies the parts.