And if you can't buy it from your local hobby shop, buy it online -- Tower Hobby may be your best bet.
A couple of notes:
I use the smallest possible tubing for 3 ounce tanks, with an ID of about 1/16". That won't fit over most clunks, so I whack off the nipple on the clunk and replace it with a piece of 3/32" OD brass or copper tubing.
Hayes makes even more flexible tubing that they sell with their tanks. It's black. I think some searching on Tower will find it for you. I also don't think it's necessary, but you may prefer it.