Ray gave you good advice. Here are several additional thoughts.

1) The illustration of your fixture on Charley's website shows the
ballasts mounted on one side of the fixture instead of in the center, so
be sure to select a similar replacement ballast to maintain the weight
balance so your fixture will hang level (magnetic ballasts are *much*
heavier than electronic ones).

2) There are 2 common types of 48" fluorescent tube (older standard 1.5"
diameter = T12 and newer standard 1" diameter = T8). Make sure your
replacement ballast matches the type of your tubes. Some ballasts are
"universal" and will work with either diameter.

3) Hardware stores and home centers often stock replacement ballasts.

John