My package says "A sweet derivative of roasted barley".
I did a quick search for substitutes and didn't come up with a lot. There is this info that's interesting from Artisan Bread Baking:
"Non-Diastatic Malt is a substitute for sugar and does not contain the enzymes to enhance the breakdown of starch into sugar. This malt is used as a source of sugar, so in a sense is it a substitute for the starch-into-sugar-converting ability of the diastatic malt. It helps the rise of the dough, but only in so far as it is a sugar. In syrup form it adds a slight tan color to the dough. Of course, being a sugar, using too much of it may reduce the rise or adversely affect the taste of the bread or both."
Another site I looked at said sugar was not a good replacement for it.
Others said it was available at health food stores.
So bottom line (at least from me) is: