Daedalus-
This is a topic that I work with everyday. I've designed all types of airbearings, and there is art and science to do it properly. The porous type seem to be the simplest type, but there are many tricks to get the proper flow out of the bearing. This website sells porous carbon and porous ceramic air bearings.
http://www.newwayairbearings.com/
Thier prices aren't too bad. These are the most modular components you can buy.
The surface needed to create a good air bearing requires precise grinding. To get the best performance , the air gap or flying height has to be less than .0005". The best performance (about 30 pounds load capacity per square inch of air pad at 60-80 psi inlet pressure) is achieved with a gap of .0003 to .0004". So, the flatness of the bearing carriage and the mating beam must be about .0001" over the area that the air pad covers. This is achievable with a properly set flycutter or a very large shell mill on an accurate mill, but usually this is done by grinding or lapping.
The material used in the graphite air bearings is a porous type of graphite used in die-sinking EDM work. I have produced these before, but it's tricky to get the flow correct. You need to use something like laquer to plug the pores if it is flowing too much air, or if there is not enough flow, there is no easy way to correct for that in a porous bearing. They are the simplest to get flat though, as you can literally lap them flat on a piece of sand paper on a granite surface plate. Just polish your way to a nice finish with finer and finer grits.
We typically produce orifice type bearings, and some designs can be simpler to produce and align than others. If you are trying to DIY, I would suggest that you make the "puck style" like the ones on the new way website. The spherical joint lets each pad self-align. You can use the stiffness chart on thier site to do some calculations for your application.
The filtering arrangement that you suggest is right on the money. If any fluid such as water or oil gets into an air bearing, it will clog the pores, and crash the bearing. Filters are cheap insurance.
Here is much more info:
http://www.nelsonair.com/NA_primer.htm
Here's a 2 year old brochure of some of the machines that we make:
http://www.neat.com/Precision-System...ns-06-2004.pdf
All of the large systems are air bearing gantry style machines (the Gen 5, 6 & 7 machines). The bases are granite, and they are all supported by passive air isolators. These are all linear motor driven, with non-contacting liear encoders, so the only connection is the cables. These can be very high speed systems, some over 3 meters per second. The accuracies over some of these large travels are also incredible, usually a just few microns of position error over 3 meters of travel.
What type of machine are you planning on building?
NEATman