Yes they are a variant of wine bags, some double skinned to reduce oxygen entry, yes 5 litre or one gallon are available ( i bought mine online from china), but quite small for a brew pub unless for off sales. I have some 20 litre ones as well.
Key kegs are good, need a different connector for them and filling a bit tricky I think from watching the videos.
I find them good with some surgery to the top and the bag ripped out as a 30 l or 20 litre keg. I just modify the top so that a triclover ball lock adapter fits on. But this works as a normal keg not bag being pressurised. OR as a secondary ferment vessel with a bung in the top and an airlock. I get them for free.
Key kegs also make great stools in a bar area with a cushion on the top but don't depressurise them and take the bag out they lose their strength and will collapse.
I was thinking these might be the best
Buy 5L mini beer kegs at Wild Beer Co. Filled with 9 pints of your favourite Wild Beer to enjoy. Grab your favourite whilst stocks last. Free Delivery Over £50
www.wildbeerco.com
small batch and they are reusable, can be bought with beer in drunk and then sanitised or bought new, lots of youtube showing how to do this. Not going to dry hop in one of these though as probably difficult to clean out.