Menards and Fleet Farm have them up here in the midwest. You should still be able to find them at local hardware stores too.
However, unless you can find them cheaply, it's often better to just buy a new fixture for $8 to $12 since you get a new tube with it.
The Sylvania branded 14-20 watt single tube magnetic ballasts used to sell for about $2.69 at Fleet Farm and $3.99 at Menards. Lowes had a clearance on them for about $1.67 a couple years ago.
Entire fixtures are about $10 at Walmart, but they may be newer electronic balasts (better--less filament wear, no flickering to start, no humming, typically work with old tubes that won't fully start in a magnetic ballast) but require you to replace more stuff.
Tubes alone are $7 at most stores, so cost/benefit/time/originality analysis to see what the better deal for you will be.
Also -- check your thrift stores. They often have old under the cabinet mounted fixtures with tubes for $2-$3. Rip them apart to get the ballast out and get a free used tube out of the deal too.