If you don't have the tax stamp on it then yes, it is illegal.
You simply file the paperwork, pay your $200, and pick up the suppressor.
It's simply retarded that we have to deal with NFA paperwork on the suppressors. It's not like it makes the gun completely inaudible. Just one of those old laws that never was modernized for the sake of tax income and regulation.
You should be able to buy suppressors with any pistol at any store. All it does is save your hearing and cut down on the noise pollution for those not shooting. However, I'll save that for another thread.
In Texas you can legally own suppressors, but you have to apply to the ATF in much the same manner as for a machinegun, pay the $200 tax stamp fee, and once you are approved you can pick up the suppressor at a Class III/SOT dealer.