Hi Jarrod, Welcome to DBSTalk. 
The 721 is only available to Dish retailers at the moment, in the future I imagine Dish Network will sell them as well. Usually the retailers are a bit cheaper anyways.
Where to buy, thats up to you. I have dealt with a couple of the online retailers without any problems. We have some that visit here.
ProCom Plus is an online retailer I have dealt with.
I can't say enough about Jason, a very nice and helpful guy.
Here's the ProCom Plus Link.
Dish Depot is another popular online retailer.
Here's the Dish Depot Link.
DishPro is the new line of switches from Dish Network, although not all are available as of right now.
Here's a Link to the
DishPro Switches.
Quad LNB is an LNB you put on your dish that enables up to 4 receivers. The switch is built in to the LNB. In your case you could hookup 1 721 and 2 more receivers.
Here's a Link to the
Quad Switch
SW 64 switch will allow you to hook up 4 receivers of your dual LNB.
There are numerous ways to cascade more switches to get 4 or more receivers hooked up.
Take a Look here at these wiring diagrams for multiple receivers
Remember one thing about the 721, it takes 2 lines in because it is a dual tuner receiver. You will have 2 RG6 cables hooked to the back of it.
Feel free to ask more questions, hope this helps a little
The 721 is only available to Dish retailers at the moment, in the future I imagine Dish Network will sell them as well. Usually the retailers are a bit cheaper anyways.
Where to buy, thats up to you. I have dealt with a couple of the online retailers without any problems. We have some that visit here.
ProCom Plus is an online retailer I have dealt with.
I can't say enough about Jason, a very nice and helpful guy.
Here's the ProCom Plus Link.
Dish Depot is another popular online retailer.
Here's the Dish Depot Link.
DishPro is the new line of switches from Dish Network, although not all are available as of right now.
Here's a Link to the
DishPro Switches.
Quad LNB is an LNB you put on your dish that enables up to 4 receivers. The switch is built in to the LNB. In your case you could hookup 1 721 and 2 more receivers.
Here's a Link to the
Quad Switch
SW 64 switch will allow you to hook up 4 receivers of your dual LNB.
There are numerous ways to cascade more switches to get 4 or more receivers hooked up.
Take a Look here at these wiring diagrams for multiple receivers
Remember one thing about the 721, it takes 2 lines in because it is a dual tuner receiver. You will have 2 RG6 cables hooked to the back of it.
Feel free to ask more questions, hope this helps a little
