Your ISP may provide you one for free, or you may be able to rent one from them. I have Charter as my ISP and they provided one to me for free when I picked up there service a couple of years ago and that was the only choice. Now, I can get one if I want to switch it out for one of my own.
For future-proofing, it would be best to get a DOCSIS 3.0 modem, preferrably one that does 8 down channels x 4 channels.
Options to look at would be the Arris SB6141, which you usually can find for about $70 on sale. There are also models from Zyxel, Netgear, D-Link and other vendors.