Leather choice

What leather does everyone use for their miniature tack? I'm looking into making some for my 1:18 scale and have tried some but it won't skive down.

Any suggestions?

For small scale tack, or anything 1:12 or less really, I prefer fake leather, adapted slightly before cutting - you can peel the foamy/fabric backing off, leaving just a very fine layer of leather-effect plastic. Skiving with half the effort and no sharp things near fingers!
This also has practical advantages : it glues better than real leather, and you can get really flat neat folds and joins for your billets and keepers. You can age it with paint effects, draw on stitch marks. And one of the best things is that it's flexible and stretchy and soft - you can glue your buckles and keepers firmly in place for neatness, and just pull the bridle/halter on over the ears without having to make working buckles and do them up every time.
I've got a couple of big sheets in black and medium brown I bought in a fabric shop about twenty years ago and am slowly working my way through, but if you want specific or unusual colours or textures then try charity shops (or thrift stores, op shops, goodwill - whatever you have in your country!) for old synthetic leather handbags which can be 'skinned' and recycled into tack.

If you're looking to use real leather that can be stamped, tooled or pyrographed, I recommend getting 1mm/1.5-2mm tooling leather. You can get it dyed or natural, which you can choose to dye yourself. I buy mine on Etsy.

I mark out my patterns on the leather, and then do my tooling or designs on them before cutting them out. Once designed, I use my Dremel with a sanding drum to take down the rear side of the leather to my ideal thickness and then cut out my pieces using a knife or very small fabric scissors.

Italian lambskin leather for doll shoes is also a good alternative if you're not looking to stamp or tool your work, it's very soft, fine and flexible and comes in all colours imaginable. Easy to skive too, with a sharp blade, I use a scalpel blade at an angle after cutting pieces out for lambskin.

People have covered thickness, I just wanted to add I use goat and sheep leather, in case you were also looking for animal-recommendations. It's thinner naturally and also more flexible for small-scale applications :)