A source for smaller quantities of steel bars for making metallophone
or harp bars is McMaster Carr, http://www.mcmaster.com/ and enter 3361
in the search field.  This gets you the page for ASTM A108, 1018 carbon
steel bars in a very large range of thicknesses and widths, including
the suggested 1-1/2" x 1/4" size.  They come in 6' lengths, which can
be shipped UPS.

According to Don Stinson, Hans van Oost and Fritz Gellerman, this
common alloy would be the best (and cheapest) choice.

Using the formula F = 209100 ( T / L-squared ), where T and L are
the thickness and length of the bar in inches, one can calculate the
theoretical required length of the bars for each note.  Cut them a
little long (flat), then grind the end shorter to tune them.

Note that the width of the bar is not a factor in the frequency, but a
wider bar would require more energy to excite it, and affect the tone.

I checked the formula from my Machine Design textbook against several
bars of my organ harp, and the results were very close.  In order to
even out the scale, Aeolian used three different bar stocks for the
bars, starting with 2" x 3/16" at the bass end, through 1-3/4" x 1/4"
to 1-1/2" x 5/16" at the treble end.

Richard Vance