You get it from LogicPD:
http://support.logicpd.com/ProductDownloads/OMAP-L138SOM-M1%28eXperimenterKit%29.aspx
You need to register your board and be logged into the LogicPD site to download it (unfortunately!).
Another option might be to use some of the materials from the TI-RTOS Workshop. Version 2.00 of the workshop has now switched from the OMAPL138 EVM to the OMAPL138 LCDK. However, if for example you grab the Labs zip file then you can go to C6000\Labs\TTO_gel and use OMAPL138_EVM_TTO.gel which should be suitable.
http://processors.wiki.ti.com/index.php/Introduction_to_the_TI-RTOS_Kernel_Workshop