I have a professional bird scale that I use. I actually use it for birds sometimes too. I bought it years ago, before I knew you could just go buy a cheaper one for home cooking. I think most people buy kitchen scales at Target or Walmart.
I got a great deal on mine from a local headshop. I did feel a bit funny when I left but I think it was just the *incense* they were burning. You can also get them at kitchenware stores if you are feeling less adventurous. I have cycled through many over the years and don't have a brand preference as they all do the same job well.
I haven't a clue what brand/model mine is. I just know I paid WAY too much for it at a candle-making supply shop. I have heard good things about this one, on a couple snake forums: http://www.reptilebasics.com/my-weigh-7001dx It weighs up to 15 lbs, so would at least last a little while for your Sullie.
It looks like Reptile Basics also has one for $6 more that weighs up to 75 lbs, though the platform looks a touch too small to fit a 75 lb tortoise on! http://www.reptilebasics.com/my-weigh-ultraship-75
Otherwise, like has been said already, just look around and see what you can find by the way of food scales.