CMMG sells receivers with short barrels already installed. The price differs depending on what optic you buy. If I remember correctly, a new receiver with the white ring is $799, and $50 more for the black ringed sight.
The stand alone receivers are not on their site (with the short barrels). But, you can call them and press the option for NFA items
The PS90's upper is its receiver. Your best route is to form 1 it and install the barrel yourself once you get the form back. ONLY after you receive the form back should you install the barrel.
If you purchase an SBR, it must be form 3'd to a local SOT from the manufacturer. Once they receive it, you'll have to fill out a form 4 to have it transferred to you. Generally, the SOT will want $100 to perform the transfer.
Save yourself the time and money, just form 1 your PS90 and install the barrel yourself. It takes me about 4 minutes to swap barrels and about 3 to swap a hider / thread adapter (includes putting the tools back)
Be aware that you void the warranty if you change the barrel.
I almost bought a seperate receiver from CMMG - already short barrelled. But they messed up the paperwork, and I had some other issues. I finally just cancelled the order after it took so long and they kept telling me different things (they even sent in their expired FFL instead of the current one, when they submitted their portion of the paperwork to the ATF).
But, I wanted to have 2 receivers - so I'd always have 1 original one.