I use standard electronic kitchen scales. However rather than flat scales, they're round ones designed to have a bowl on top. I use the bowl that comes with them for smaller rodents, and a bigger diameter ceramic bowl (thank you tare function) for the piggies.
I need to buy a new battery though as it's gone flat and I keep forgetting.... I would say it's worthwhile in the long run to buy scales with a relatively normal battery requirement rather than needing those flat round ones.