I recommend the Metzer YouTube video on vent-sexing ducks:
(To be honest, I don't do this, myself. I know that I will get about 50/50 boys and girls, and I don't sell them as one sex or the other until they are older, so I don't need to know until then.)
I also only band them when I take them to shows. I use a spiral leg band, so I can take it off again when they get back home from the show. But my friends who raise meat ducks and chickens use zip ties. You have to make the zip loop large enough to accommodate the full-grown leg, so you don't cut off their circulation. This is also true for permanent bands, which must be put on when the bird is young enough to put their foot through the band.
You will want to look at the band on the bird regularly. Mud and dirt can cake around it and cut off the circulation, and if they free-range the leg band can snag and injure their legs. For me raising backyard layers and show birds, that would be a disaster, but I know that's not such a big deal for meat bird raisers.
You might also consider wing bands. They are nearly invisible, so not as easily read as a leg band. But they are also less likely to cause an injury or fall off.
As far as I know, National Band (
https://nationalband.com) and Ketcham (
https://www.ketchummfg.com/livestock-identification-tags-metal-plastic/poultry-leg-bands-wing-bands/) are the main suppliers. You'll find some types of bands at most poultry supply and hatchery online stores, which is usually where I get mine.
