Seltzer tablets are gross imho, but if you're handy you can assemble your own commercial grade carbonator from standard 'industrial lego' parts that will accept standard CO2 bottles (not the proprietary crap soda stream uses.) It's an up front investment but if you drink a lot of seltzer, it won't take long for it to pay itself off. With a little bit of home DIY/carpentry you can make it look nice too.
If you go to a local restaurant supply store they can likely guide you through the whole process.
If you go to a local restaurant supply store they can likely guide you through the whole process.