FX Impact foster fitting thread size?

Google a 1/8 BSPP double female. You'll get a few results. Or, check with any of the major retailers. 

CAUTION! - Make sure what you order is rated for the high pressures you'll be using it for! 
I learned the hard way by buying about 10 NPT to BSPP adaptors that were under-rated. Luckily, I found it out before any accident occurred!
Also, BSPP is different than BSPT. So, watch out for that too.