I have no clue about what the Peoples Republic of New Jersey will allow you do do with the stock. Logically, if you're replacing a pinned cheek piece/stock with another design and it's also non-adjustable, I'd think that would be legal. But you need to check with the NJ State Police as logic and statute law aren't necessarily the same. If legal, you should be able to fix the stock length where you want it.

As to sources of stocks that will fit commercial tubes, check out Brownells. Midway might also have some. Mag-Pul makes a bewildering array of AR accessories. If none of those float your boat, then changing the receiver extension to a mil-spec one is possible. But, the extension has to be installed correctly. Look at how the current tube lines up with the spring loaded recoil spring retention pin before you remove the current item. A replacement extension has to go in the same. You'll need a spanner wrench to remove the castle nut.