Spare parts for M&P15X

[bryman021]

I have a M&P15X, and want to stock up on spare parts. I cant seem to find any place that has S&W spares. Being that the parts should interchange with other brands would it be ok to use say STAG, RRA, or Bushmaster parts?

The main parts I'm looking for are:
LPK (Lower parts kit)
Firing pin
gas rings

 

[arizona98tj]

The M&P15 uses standard parts.

I currently run a fire control group and bolt carrier group in mine that didn't come from S&W. I tossed the removed factory components into the AR spare parts box.