I use something that Advantage sells...but if they don't want to pay for sewing, how in the heck are you just "throwing in" the time that it takes to hem and grommet 30 banners? You should in the future just build that into your cost, and don't tell them how it's being done.
If I have to have mine hemmed, I have a local canvas shop that will do it, but I don't tell people they're paying extra for it, I build it in to my cost, and sell them on a banner that's going to last a little better. Plus, I don't have a ton of time wrapped up in hemming them.
I did a big order like yours once, and it was cheaper for me to print them and leave them on the roll than to mess with hemming them. It was indoors, but the canvas shop did all the trimming, hemming, and grommets, and I couldn't afford to mess with it for how fast they could do it.