"Office 2013 with SP1" as an ISO, is available from VLSC. But that's basically the RTM with the SP1 MSPfiles unpacked into the \Updates\ folder, last time I checked ;)
The Office team abandoned the concept of slipstreaming or re-baselining, a long time, AFAIK.
We also use Office2013ProfessionalPlus, and have a similar as-yet-unsatisfied challenge.
Office365ProPlus C2R doesn't suffer this problem, if you grab the monthly "build" via download and then deploy that, instead of a previous "build".
I don't have a great answer for you, but there are a few ambiguous/confusing/contradictory articles around the web, which may enlighten you a little bit.
It seems to me that for the Office client software suite, the Office team uses words like "cumulative" and "rollup" in slightly different ways to other MSFT product/engineering teams [e.g. the Windows team, the SQL Server team]..
http://blogs.technet.com/b/gray_knowlton/archive/2013/08/07/how-office-updates.aspx
https://support.microsoft.com/en-us/kb/824684
[Description of the standard terminology that is used to describe Microsoft software updates]
https://technet.microsoft.com/library/dn789213(v=office.14)
[After the December 2014 CU, all updates will be included in the PU. CUs will no longer be released.]
http://blogs.technet.com/b/office_sustained_engineering/
If you figure it out, let us know ;)