Check office supply stores and kitchen (cooking ware) stores. Watch for sales!
A digital or balance scale that weighs up to 5 lbs. to the nearest 0.1 oz. is best. If it also weighs in grams, it's nice for communicating with folks from other countries (since the US is about the only country not on the metric system) or for cooking from European recipes. A tare function, it saves some math. The tare function lets you zero out the weight with a box or a stuff sack on the scale so you can weigh bulky items (sleeping bag, for instance) that won't stay on the scale by themselves. Neither the gram nor the tare function is strictly necessary. You can convert to grams (lots of tables available on the internet) and you can do the math to subtract the weight of (for example) the stuff sack from the sleeping bag.
I can't recommend mine (Pelouze) since in the 5 years since I bought it the price has almost doubled! It's an excellent scale, but hardly an economy model! But there are many other brands.