Hi matey, I personally bought a mechanical gauge (instead of electrical which uses a sender.) I have my gauge fitted when my radio was, so simply routed the copper oil feed pipe through an existing hole in the bulk head towards the back of the head. Here I removed the sender and replaced it with a T piece, which the sender goes into, and also the oil feed pipe to the gauge. To power the illumination of the gauge, I came off the illumination ring for the cigarette lighter! If i remember correctly, I searched around alot (i.e prices etc) and it ended up being easiest, and sensible money to just order one from Demon Tweeks.
http://www.demon-tweeks.co.uk/performance/gauges/tim-oil-pressure-gauge-mechanicalThat is the gauge I have, looks like its gone up a few pounds since I bought it. But for ease, when I bought, it came complete with all fittings etc.
Hope this helps matey