Upgrade and purchase a new Modem

Good news is, you don't need either of those two modems, nor are they for sale.  If you scroll down that approved modems list there are plenty of other options available sorted by tier of service which can be approved from a local retailer of your choosing.  I seem to recall you mentioning having a CM8200.  That's a perfectly good modem for speeds up to Gig.  It's basically the Cox provided version of the SB8200 so unless you're paying a rental fee I see no reason you would need to change modems.  I believe you can even still purchase the CM8200 you're using if you're currently renting it.  You might have to call in and inquire about converting it to a purchase.