You can now download Sharepoint Foundation with SP1 integrated from Download Center:
A couple of people already tried to create their own slipstream version using RTM and SP1. Please avoid doing this as this is unsupported due to a change in the package layout which was introduced with March 2013 PU. Due to this change in the package layout it is not guaranteed that the manually created slipstream version will update all modules correctly. Ensure to download the official SharePoint Foundation 2013 with SP1 from download center.