I went out last Friday morning to have a look at these guys:
http://www.metalmesh.com.au/stainless-steel-mesh
They are a manufacturer and wouldn't give me a price (not even a ball park figure) which was a bit frustrating as I now have to go to one of the retailers and waste their time getting a quote which will probably be way too high and a waste of everyone's time. I went in and a receptionist gave me some sample books and left me to it. So I didn't even get to talk to anyone technical about materials, strengths, weaknesses, try and con them into giving me some off-cuts, etc. Anyhoo....
If you have a look at the "hollander weave woven wire" link on that page, that looks the most suitable. You would still have to build actual framework to stop it from bending under the pump pressure. I think you might get away with not requiring filter material as the weave is quite tight. I got one of their brochures which has photos of the available metal at actual sizes. I also wrote down the two choices I thought would be suitable so will go to one of the retailers and get them to quote it up for me. But this is all at home so can't quote product codes. I'll post them up tonight.
None of their perforated SS sheeting looks suitable as the ones with the smaller holes is made from a thin sheet which I could bend by hand, and the thicker sheets had 5mm holes.