This is a list of all the different vendors I've been able to find who sell binders or inserts in the M5/Filofax Mini size. This size of planner uses paper with five holes spaced 19 mm apart; I've linked some compatible hole punches at the end.
I've listed sites by category, and linked to the narrowest category available on the website; as they differ in how granular their filtering is, I've sometimes linked the same place under multiple categories, and sometimes listed the same site several times in a given category. They are not in any particular order, but I've made an attempt to list sellers with more products first.
Important note: not all inserts are the same width! Most M5 inserts are 67 x 105 mm, but Plotter uses a size of 62 x 105. If mixing widths will bother you, make sure to check which size you're getting.
Binders
Filofax (also on Amazon, where they have more designs available)
Catspresso Co
Plotter
Moterm
MeePlus
Raymay Fujii (via Yoseka Stationery)
Thinkthankaymade
libertaz
ZackLeatherStudio
Techo Treats
ChicChoi
Ashford (in Japanese; google-translated version)
Ashford: refill storage binders (in Japanese; google-translated version)
Pinkoi (link goes to search results; it's a marketplace for multiple vendors so lacks good sorting/filtering)
Aliexpress (link goes to search results; it's a marketplace for multiple vendors so lacks good sorting/filtering)
Printed Inserts
Filofax
SortStuffOut
Catspresso Co
Catspresso Co: sticker paper inserts
Plotter (plotter size)
MeePlus
Raymay Fujii (via Yoseka Stationery)
Desk Gems
Troncle (via Pinkoi)
ItsSimplyBizarre
ChicChoi
StudyJoy (note that some of the preview images appear to be AI, and I haven't yet purchased any to double-check) Doesn't currently appear to be selling
Ashford (in Japanese; google-translated version)
Aliexpress (link goes to search results; it's a marketplace for multiple vendors so lacks good sorting/filtering)
Dividers, bookmarks, and accessories
SortStuffOut: dividers
SortStuffOut: dashboards
SortStuffOut: decorative printed vellums
SortStuffOut: bookmarks
NennyLife: dividers and bookmarks (I've linked to her site on UK Ebay, as categories aren't working on the US one)
Catspresso Co: dashboards
Catspresso Co: decorative printed vellums and acetates
Plotter: accessories (plotter size)
MeePlus: accessories
RocketTreasures: rainbow dividers (plotter size)
DannysPlannerCuties: pink/purple watercolor dividers
DannysPlannerCuties: seashell dividers (he only has two dividers available in M5 size)
Ashford: accessories (in Japanese; google-translated version)
Aliexpress (link goes to search results; it's a marketplace for multiple vendors so lacks good sorting/filtering)
Printable Files
CreffectivePaper
123ZenDesigns
HausOfChibs
SortStuffOut
SortStuffOut: printable dashboards
HappyGhostPlans
PlannerRoomShop
Thinkthankaymade
ListLab
LofiNote (plotter size) Doesn't currently appear to be selling
DIYfish
StarCreativeStudioUK
Hole Punches
I recommend the KW-trio 9027, which makes the smallest holes (4 mm diameter), and has three punch dies rather than 6 (useful to avoid inadvertently punching the tops of bookmarks). You can find it here on Aliexpress (currently $10) and here on Amazon (currently $15).
Filofax also sells hole punches, which can be attached to the rings of your planner; the Pocket size should also work for minis.
For a more durable option, there's also the Rapesco Adjustable 6-hole punch, which is all-metal (and rather heavy; even aside from the smaller holes I prefer my 9027).
Closing Thoughts
And that's all I've got! Of the stores I've listed, my personal favorites are: SortStuffOut (great variety of binder decorations); CreffectivePaper (hands-down the best variety of printables, and offers a rare vertical weekly layout with hour markings); CatspressoCo (offers a vertical weekly with hours in printed format); NennyLife (has some lovely divider patterns); Filofax (I love that their daily layout has little month grids on it, and in general appreciate the density of info); and Aliexpress (hands-down the best place to find cheap binders, and they even have knockoffs!). Also, shoutout to HausOfChibs and 123ZenDesign for the delightful gothic font and wide variety of delightfully colorful layouts, respectively.
I hope this post helps you find the perfect supplies, and happy planning!