Any brand of digital kitchen scale that weighs in grams and has a "tare" function, which they all do anymore, is perfect. Really no need to order one online unless you find a tremendous deal, as you can run to your local Walmart, Target, Bed, Bath, & Beyond, etc. and buy any number of them for between $15-$30. I recently bought a second one at my local Walmart because I decided I wanted to start weighing my Bearded Dragon as well as my birds, and I wanted a separate scale for my Beardie. Walmart probably had 5 or 6 different digital models, all weigh in grams, all have a tare function, the only differences between them are the size of their platforms and whether or not they have a memory function. The cheapest one was $14.99, and the most expensive one was $29.99, and it had the memory functions and some other functions that would only be used for cooking.
So you've got a lot of options available locally to you, but if you find one online that is really a good deal then that's another way to go, but I wouldn't pay for shipping when you can just run out any buy one locally (unless you aren't able to drive, etc.). There's nothing special that you have to buy to weigh your birds, I don't know anyone who has purchased any expensive, scientific scale to weigh their birds, lol. Pretty much everyone uses a simple, digital kitchen scale.