I was told I could come in and pick up the box for wifi and hook it up myself, can I get the box there?

Sometimes. It depends on if your particular address is eligible for a self install. Most are, but not all.

Make sure you make an appointment to get any equipment..

I agree with Paul. Most likely you can do it yourself, but not all addresses allow it. Also, be sure to ask Comcast about whether you'll need a splitter on the line to connect both WiFi box and a DVR.

There are modems available for rental at the store