you might try Granger or McMaster-Carr.
I also have MG-1. This filter is a low restriction filter, so affect on air delivery, as I experience it, is minimal. It does not matter where the filter is, the best way is before the pump, but there is no visible air inlet on the pump supplied with MG-1, so if I find a way to move the filter there, yes, it would be a logical way to do it. Yes, this filter kepps dust away, which eventually will clog up those tiny holes in MG-1's air tube. Another thing I would put there (and probably will, as NYC is a humid place) is a water trap, which I found on line for about $15, drainable. Also, a nice air tank to smooth out the pulsing (after the pump). And to get really sophisticated, a pressure regulator! BTW, these fuel filters can be had on line for as low as $1.50, but I decided to go to my local auto parts store and actually blow into it and see how restrictive it is. Mine has a paper element inside, similar to large air filters for carburetters and fuel injection. These are the part ## that Bruce suggested:
Pronto - 33011-wix
Mine is a WIX Filter part #33066. No direction flow, nipples are slightly too big (the smallest available, 1/4"), so you have to warm up the clear line's ends carefully with a lighter and they go over easy. Hope this helps.
Maplenoll and its various advisors, Lloyd Walker, Pierre Sprey, recommend putting all doodads close to the pump followed by 500 feet of hose as final smoothing of pulses, resonances and turbulance caused by juncture points, etc.. They use a fuel filter on the input of the pump with one or two plenums after it, followed by a carbon filled water filter followed by the splitter and control valve for the platter and arm. I've added two small water/oil traps, one right after the pump for the oil and one several feet of hose downstream to allow for further condensation as it cools. You can make plenums and water traps with 6" PVC pipe and, end caps and screw in nipples. Fill the plenum with polyester fiber fill.
It does make a difference where you locate the filter. Keep in mind that if you use a surge tank (as I and many do with their ET's) that the tank is filled with fibrous filler. It is possible for some of this, and possibly contaminants (oil?)from the pump itself to dislodge and work their way to the air capillaries of the arm. So, it seems to me, the best place is AFTER the pump, and if using a surge tank, AFTER that.