Do you all know where i can get sparepart for my Seagate Barracuda ST3120213A, some of its component like QE r615 next from power connector burned?

Your best bet would be to go on e-bay and try to find the same drive for sale. The tricky part is swapping out the board from one to the other. You'll have to experience with de-soldering and soldering. You'll first remove the logic board and or connectors from where its connected on the "new" drive you purchased from ebay, and remove the logic board and or connectors from your old drive. As you can see from the picture most of the hard drive logic boards are secured with torx bit screws so you'll need a torx screwdriver also. All hard drives are slightly different so take your time and inspect how the drive comes apart before you disassemble it.
If all else fails you can take your drive to a computer repair shop for data recovery, but be warmed... this process is usually very costly and there is a very good chance that your drive will be destroyed in the recovery process, hopefully it wont come to that though.