Originally Posted by
LV2TNDM
This is undoubtedly a 14g, or 2.0mm straight-gauge 264mm spoke. That's what you need.
However, this almost certainly will not be the only spoke to break. Spoke failure like this in an old wheel signifies the spokes are at the end of their fatigue life. More will break and probably soon. There's very good chance you break another spoke first ride after replacing the one that broke. Or even before this while truing the wheel during spoke replacement. But there's only one way to find out. Replace it and see what happens. If you've already broken a spoke (or more), then you already know.
The solution is rebuilding the wheel. Given its age, you'll need a new rim, spokes and nipples of the appropriate length for the replacement rim (and lacing pattern if you change it). For the best wheel, switch to 14g & 15g butted spokes, or 2.0/1.8mm.
Not sure you need a new rim. Lay your ruler across the brake surface of the rim -- if there's a gap where the brakes have worn the rim surface down, yep, you need a new rim. Otherwise, just swap the spokes -- either all at once, or one at a time, your choice. Though it's more trouble up front, for the aggravation factor, all at once might be better.
If you do need to replace the rim, it's probably worth taking it to a shop to order a new rim and matching spokes. With a good mail-order source, they can help you figure out what you'll need, but get out your calipers to measure the hub.
Concur on the 14/15/14 double-butted recommendation. It's a little more than straight gauge 14 spokes, but it's easier to build a durable wheel with the double butted, and they're 1/3 the price of triple-butted.