All the electronics parts/spares shops in Doha are pretty much located in one place. They are on the south-west side of Souq Waaqif (Iraani Souq) and directly the west side of Souq Najada (the "mobile phones" souq!). The prominent among them is Bashir Radio (a shop which was established a real long time ago, and is synonymous with electronics spares in Doha).

Here is the annotated Google map of the shops.  Basically, just go to this area, and ask for Bashir Radio.  There are some other shops next to them, all dealing in electronics spares.