well, they probably have standard 1/2" threads, so any NPT 1/2" threaded barb should fit it. does the instructions sheet say what size the threads are? you can always do what i did - take the thing to home depot and start screwing barbs in to see what fits
but try 1/2" first. it might be a little hard to get in, so instead of using a metal barb to test fit try using a plastic one instead. and when you do get the correct barbs, pick up some teflon tape too; you'll need it to seal the barbs (dont use stuff like goop on the barbs, use teflon tape only).