ok, here is what i have done......
the filter i bought is from a Graco Contractor 2 airless paint spray gun.
They were the only pre made filter tubes of the correct size i could find but they are stainless mesh, available in 50,60 and 100 mesh filters. they are very cheap to buy, in the US and the UK. they do make other style filters which may be more suitable but as i couldnt physically see them all without buying i went for the style that i thought would work best. i only actually used the mesh filter and chucked the rest away, but for a couple of bucks so what!
http://www.ebay.co.uk/itm/Graco-Air...849&pid=100015&prg=1006&rk=1&sd=130340860596&
if you cant get them local over the counter, they do come in packs of 2,5,or 10 i think.
ok lets start with the petcock.......

with the OEM filter removed (mine was missing anyway) you have 2 holes in the top. main tank, which is 8mm diameter and reserve/prime, which is 6mm diameter.
you need 2 sections of pipe, st/s,steel,aluminium,copper,brass,plastic or whatever you got. i used brass thin wall.
i cut them so the main tank tube is 50mm and the reserve tube is 5mm from the mounting face of the petcock. you can alter these lengths depending on what size tank you have and whether you want a larger main tank or bigger reserve range etc.
they are a very tight fit in the petcocks but if you want to be safe then apply a drop of bearing fit to secure them.
i then added a piece of fuel resistant heat shrink tube to the 8mm pipe so the filter is a nice snug fit......
depending on the size of your main tank tube you may need 2 filter mesh's for the petcock filters. i used 1 for the main tube, which sits about 85mm from the petcock and cut a smaller piece from another filter, so it sits about 20mm high. push them into the petcock, over the metal pipes. again they will be a tight fit and the reserve tube mesh distorts slightly but it wont matter.
i then found 2 plastic "caps" lying in one of my "miscellaneous" boxes to cap off the top off the filter mesh's. secure them with a small blob of epoxy resin.
and there you have it, a very cheap replacement for the petcock filter which is strong,washable and will probably fit most petcocks i guess.