I get mine if I mail order from SAMS in the UK:
http://www.samsmodels.com
However I still have a good old fashioned (I mean really old fashioned) model shop. I've been to it since I was a kid, I used to go in along with another one which closed down 15 or so years back (this is a small market town which had TWO privately owned model shops!)
It's run by two old folks, they must be in thier late 70's/ 80's. He looks after the Hornby trains, she looks after the general model stuff. She is very good at selecting balsa and I go down there on my lunch every now and again and but half a dozen sheets or so, it only costs less than 75 pence ($1) a sheet, and she just stands there patiently as I pick out the best sheets. Anyhow they still stock the Banana oil, only in little glass jars, but to be honest I'd rather pay the few extra pence and help keep them in business, I'm sure that in a few years it will be gone.......