You need GE44 or #47 bulbs... these are the standard bayonet type.
The perspex can be bought from any sheet supplier... look in you yellow pages under SIGNS.. will have a few local suppliers of acrylic sheet.... standard size is 1220 X 2440mm though, so unless you want a lot, then find a sign guy that will cut you some down.
Youll need 2mm thickness to exactly replace, but i often use 3mm
Most sign writers/plastic suppliers will usually sell offcut pieces, usually large enough to make several plastics from, without them having to cut a large sheet (which will cost more). I always use polycarbonate plastic sheet to make pf plastics, as i always thought perspex would be too brittle. I DID find that to be the case when i started trying to make my own plastics, using perspex, i've had them break while trying to drill them for the post screws 
Polycarbonate seems to be more durable.
Theres a knack to drilling plastics.
use a blunt drill bit, or a masonry bit, or use drill in reverse, or place sheet on top of some MDF etc and drill through that way - VERY carefully.
What happens with a sharp drill bit is that it grabs as it goes through instead of cut.. this causes cracking.
Just at the point of the bit pushing through, you should be drilling very slowly with no push on your part.
But in all honesty, a blunt bit works best, if i HAVE to use a new bit, i run it up a brick wall first whilst running the drill... just to take the very edge of it.
Youll find any platics worker will have a collection of blunt bits... gold dust to us
I usually place the plastic down, as you say onto some MDF to drill the holes, while holding the plastic down firmly, and very little pressure on the drill bit. And you're right Gavin, blunter drill bits do work better.
lately I've been using a Dremel to do the holes, using a normal drill bit to do a starter hole, then enlarging with a dremel and a small grinding bit to enlarge to the correct size. Care is still needed so as not to slip and scratch the surface of the plastic with the Dremel bit!
