Got the liquid from MDP.
http://www.mdpsupplies.co.uk/productdetails.asp
I like it because you can be more specific than the spray. Pack of artists brushes from the pound shop and you're laughing :D Of course it can take longer to do larger areas with a brush than a spray.
Just tested some flock and it's left a mark but no backing, just the same as vinyl. I find it depends on the the colour of the garment and the vinyl used if it leaves marks anyway.