Lots of two-pole breakers have just one handle, common-trip. The SqD QO series is one example. Since two circuits on opposite poles share a neutral, shutting off just one of those circuits leaves the other circuit energized. If the energized one is actually supplying some load and the neutral is opened at certain points, as one might do if working on the other circuit and believing that is not live, the open end of the neutral nearest the load of the still-energized circuit will then be energized at 120 volts. Nasty surprise- or worse. The common-trip or tied handles solves that problem.
Your store can order the breaker. Will they? Ask them.

Here is a link that might be useful: Breaker

Thanks for the link, Bus_Driver, but the Home depot stores near me don't sell the item (Enfield, CT, Wallingford, CT, West Springfield, MA, and a few others). Looks like I'll have to order through Amazon.com, for a few bucks more.

I had no trouble finding the same breaker a few years ago, when I put in a well pump (240V, didn't have a choice about the trip handle then...)

It used to be that the handle tie was only require if you fed the same yoke with both sides, now it's required in all cases. The GE 215 breaker isn't that hard to find...You can order it online and have it in a couple of days if you can't find it.

Approved handle ties are still allowed and are a less expensive solution. Since you're having trouble finding a common 2 pole breaker, then you'll probably not find a handle tie at the big box store. I'm guessing that part of the problem is that this breaker might be one of the 1/2 size GE breakers.

You can always call some electrical supply houses in your area, surely there is a stocking GE distributor nearby.

tom_o :

Yes, it is a half size breaker. Are these becoming obsoleteted/ phased [no pun intended] out by GE?

No, they're still available and in common use. Can't account for why your homecenter doesn't have them, but they are available online as I stated.

Alternatively you can move some of your single pole full size stuff from higher up in the panel down to single pole skinnies and put a full size double pole up top.