For knobs, i found similar ones like the picture in the left here localy and paid 5 euro for 6 pcs. If you can't find them locally, there are plenty of places to search on internet. here is a link on ebay for these knobs. I used the mouse wheels encoders for : Altitude, Heading bug, Gyro drift, VOR1, VOR2, ADF hdg. but if you have more of them is not a bad idea, maybe you'll need them later when building ther radio stack and the gps unit.
Knobs on e-bay  $4.50 for 10 pcs (you only need 6) including shipping.