You can buy them from hunting and outdoor gear stores.
They are used by Treestand hunters.
Mind you they are very destructive to trees.
They punch through the bark and allow bugs in if you remove them.
Tree gaffs are the things you strap to your feet.
A general rule is never gaff a tree you want to keep.
For either of the following, try a good big electricians supply house,
like maybe Graybar.
Called a Hook-head Step Bolt. Try
For the strap-on climbers used on poles/trees without "hooked steps"
For the drive steps that hammer into the pole, you want these from
Chance Hardware, now owned by Hubbell:
available through your local electrical house.
You'll need to drill a pilot hole, and then they have a threaded shank
with a ramp-shaped one-way thread - the steps hammer into place with a
hand sledge, and then you turn them to get them straight up. And they
unscrew out if you ever need to remove them (put a dowel plug in the
hole or you leave bugs an inviting way in).
Use the removable steps if they are within 8' of the ground and you
want to keep children and fools off the pole. They have a special
double-head nail and backing plate on the pole, and you bring a set of
removable steps that hook onto them for use.
If you want climbing gaffs, follow that link for Klein Tools. There
are different gaff lengths - short for climbing poles and long for
climbing live trees. But be VERY CAREFUL to wear all the safety gear
(gloves, hard hat, safety belt and pole strap, long sleeves, long
pants, proper boots, etc.) and get an expert to teach you how to climb
on gaffs. Using gaffs is counter-intuitive and not easy, with a very
high price to be paid for any screw-ups, even small ones. Been there,
and came thisclose to falling a few times...