Are you wanting thread size G 1/2 (20mm) like what is spec'ed for the PFA-135 box shown?
If so, that's BSP (British Standard Pipe) size and is nominally equivalent in size & pitch to the U.S.'s NPT (National Pipe Thread) size of 1/2".
lol, I work for a large P&H & HVAC distributor. You aren't doing a local wholesaler any favors coming in off the street to buy $10 worth of material. I have found 1/2" and 3/4" fittings like you describe on Amazon as well as 'real' Sealtite in 3/4 and 1" flavors - there are vendors that sell a kit of 25 or 50' plus a handful of straight and angle connectors.