If you find a mechanic who allows you to supply parts, don't be disappointed if the part is flawed and he tells you it's now your problem because it is - you supplied the part.
You get to hassle with returning the item to the supplier and exchanging(assuming they will?), then returning with a new part.
Now the question arises - are you going to pay EXTRA labor for the tech to start over?
It could happen.
As Steve Brotherton mentioned in another post, parts pay the labor bill. If a customer is supplying parts, how can the shop make a living? I've seen this practice used, but only by very small operations and it's not likely they allow it to become a common practice.
1991 300-SEL - Model 126
M103 - SOHC
James Madison - Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace.
Liberty Firearms Training - After years of trying to regulate California criminals with a never ending stream of gun control laws they have still not given up with the belief that if they make something a crime that criminals will miraculously start obeying the law.