You may acquire some Symbol Barcode reader (or any other from whatever manufacturer, many of them use the same chipset from Texas Instruments and are therfore compatible), they use to come with API, sample code and documentation.
or...
You could use one webacam (or the camera of your choice) and take a look into this project:
http://sourceforge.net/projects/datamatrixnet/[
^]. It's a port from library
http://www.libdmtx.org/[
^].