They are MSR Fuel Canisters you can get from REI. Just log onto their site and they've got a couple sizes. They work perfect on the Ventura Rack, plus they will fit in the RKA saddlebags too.
Used them while doing the Iron Butt Rides. Plus always carry them while leading tours for my guys with tiny tanks or forget to fill up.
We also carry siphon hoses , but these are the easier.