Since you asked what scale to use, the only thing you really want to look for is a scale weighing in grams and with 1 or ,5 increments. thats basically the only thing you need to look for to be honest. I picked my scale up for about 7USD and then added a perch to it.
link to the guide I made while doing it:
http://theparrotforum.com/viewtopic.php?f=22&t=9039&hilit=diy+scale
and if you read the coments you get other ideas too, such as making it a Tperch instead (plenty of guides out there how to make a table top perch, that you then could set on the scale. Michael posted in the thread a link to where you can get his pocket perch wich you can place on the scale...
And well if you have a smaller bird you wouldn't really have to use a perch for it since the feathers wont touch the table next to the scale, well if you get a big enough scale you wouldnt need to with a bigger bird either...
The problem is that bigger scales usually handles bigger loads and for some reason the price tag goes up extremely much in order to get the same accuracy at low readings as the cheap smaller scales.