It is true that MBRP does not manufacture an x-pipe. Currently, none of the other brands x-pipe will be a direct bolt up match for the MBRP headers. Your best bet will be to have an x-pipe custom made by a local exhaust shop to match the MBRP headers to your catback.
Another point of note is that in MBRPs testing, an x-pipe offered no power gain over the h-pipe. Also, when couple with their catback, the x-pipe did not produce an acceptable tone. Hence their reasoning for sticking to an H-pipe.