Are there MoP 5.4.8 Cores out there? Absolutely, there are core emulators for every Wow Expansion, mostly the last patch.. but nonetheless, every expansion is covered.
Now this is the unfortunate part, "are there any WoW 5.4.8 Cores out there that are fairly decent?" to which i must admit from personal experience.. No, the higher the expansion the lower the over all core emulator quality. This is probably your best bet.
Project Skyfire: ProjectSkyfire/SkyFire_548: SkyFireEMU is a full featured World of Warcraft: Mists of Pandaria emulator written in C++. || Compatible with World of Warcraft client 5.4.8 (Build: 18414) Project Established in 2011 || Support on Discord https://discord.gg/DnKZycD (github.com) It is the only Open-Source MoP Emulator that still is being updated, atleast that i know of, Or you could attempt to contact any of the PServers attempting to release the so to speak "fresh" MoP Experience. Who knows? Maybe you'll be able to buy the source (i'd just go with the open-source solution and help out there instead, but you did ask for options).
All in all, anything above WoTLK will degrade in terms of quality and functionality. It's also harder to find for free and you'll most likely end up just crawling back to Wrath anyways.