I found an MPP MkVI in an Antiques Emporium a few years ago, and had a lot of pleasure experimenting with it. I was surprised as to where it led me! (Like purchase of gauge blocks and learning about precise mechanical measurements!)
Lots of good advice already! Specifically, Mifsuds are offering 5x4 darkslides at the moment. If you phone them, they may offer you a discount for quantity. And then you have to learn how to load them.
I started with a Polaroid Film holder, but seem to remember that on my MkVI I had to do some slight modification to get it to fit correctly. In addition to Mifsuds; ffordes and MXV are worth checking out for lenses, lens panels, viewing cloths etc. I bought the last 5x4 FP4 from Jessops in Milton Keynes, and some Provia 100 which was nominally passed its expiry date. My biggest problem was forgetting about my DSLR mindset, but I did get some memorable results, and now have a lot of respect for those who are expert in it!
(If you seek out extra lenses etc., do use the MPP Users website for clarification about the 3 different lens panels that MPP cameras take.)