Clippards have some fans, but I've had 2 or 3 of them fail within 6 months. Plastic LFS valves are useless past the first few weeks. The "stainless steel" silvery ones with the arrows, originally from China (check Evilbay, they're now resold practically everywhere) might last a few months. Of all these, Clippard is the only one you'll find that has NPT threads for attachment to the regulator rig.
Parker and Swagelok, and likely the same companies that provide functionally identical compression fittings (there are at least a dozen - Bi-Lok, Tylok, Hylok, etc.), will have top-of-the-line check valves at 1/3 and 1 psi. That's about the best you can get for our purposes. For a valve with NPT threads, these are the manufacturers you're looking for.
I have a couple Swagelok brass and SS inline (compression, not NPT) check valves for $32 or less, shipped with inserts. Find my sales thread for a bit more info. Otherwise, Swagelok is the place to get them with NPT. SS-2C2-1/3 or B-2C2-1/3 are the models you'll be looking for. Parkers are a good deal more expensive.
P.S. Eventually you're going to dump the Aquatek for a decent industrial rig. Keep that in mind when deciding which pieces to upgrade over the next year or two.