I ordered c-stock and magtube from Germany (I'm in Sweden), took 2 weeks and they didn't even have the c-stock in stock at the time my order.
https://sportwaffen-triebel.de/en/benelli/m4-super-90--pistolengriff/teleskopschaft
Ok prices and very good customer service. But I guess some kind of import fee will be added for you guys in the US.