A:
First, that is a Motorola SB6121, and it is a cable modem, not a router. BTW, it is a really stable, excellent cable modem!
(I bought the same one at Best Buy myself and it is an excellent, very standard product. The M at the top is for Motorola and I can read the SB6121 at the bottom! It is better known by that designation.)
Second, they sell it at Best Buy and when I bought mine I price matched because it was available at Best Buy online cheaper, so do check the price.
Third, you can connect it to any router and they'll all work fine unless you have higher needs, like streaming multiple movies at once and so on.
Cheap wifi routers run $25-50. You don't need a router if you just plug it directly into one computer using an ethernet cable (with the oversized phone jacks on each end "RJ45", comes with the modem.)