I find it hard to believe that a bike shop didn't sell bike locks.
Regardless, looks like the Dick's in Latham has this one for $35. No-name brand, but looks decent enough.
But if you can afford to wait I'd recommend getting a Kryptonite. They're simply better locks, both sturdier to better resist brute-force attacks and better locking mechanisms to be less easily picked. REI has the one I use (free shipping+dividend if you're a member), or you can pay a bit more to get it from Amazon.