I don't, no, never seen one anywhere, but I'd probably buy one if there was. Maybe you should patent the idea!

I get round this at the moment by using old 1 and 5 litre water bottles which would otherwise just go for recycling. I make the mix up and use it straight from that, or for larger amounts, make up two lots in the 5 litre and pour them into a 10 litre can, though its rare for me to do that - 10 litres is pretty heavy. If you're wondering about having no rose on the bottle, I always remove the rose from the watering can anyway when watering pots and containers - I don't want it sprinkled on the leaves, I want the water on the compost in the pot.

Or you can buy large 2 litre graduated plastic jugs from supermarkets. I would be lost without mine and use it for heaps of things beside filling wateirng cans of plant food- even for washing the car - a bucket of clean water, dip out a jugful to use and refill when its gone - the bucketful stays clean. Same thing for cleaning windows outside. And I fill it with peanuts to put in the bird feeders, and for watering house plants, etc etc....

Thanks!