3.5mm panel-mount socket
mouser.co.uk = £1.01 each
uk.farnell.com = £0.71 each
Other sockets should also fit, as long as the threaded bit will fit in a 6mm hole.
We recommend the 3-pole/TRS sockets as they allow the use of standard stereo mini-jack cables, but equally you could also use 2-pole/TS sockets, or 4-pole/TRRS depending upon your own requirements and instrument.