Mommy Charms used to be painfully expensive for what they were *grumbles to herself on the prices spent on them back in the day*. However a few years ago hundreds of in packet ones were found in a closed down factory and they flooded ebay in massive numbers. That meant that their prices fell down and they have never really recovered. You can pick up most of them for anywhere from $6-10 each.
There are some which pop up a little more often than others but not to any degree that there is an obvious handful which are common and some which are super rare. However a lot of the purple, blue and white ones can be prone to very bad age marks and they can be harder to find in minty condition.
The leaflet was boxed with almost all the ponies which were released around the same time so its not that hard to find. But it does not come up for sale as often as the charms themselves.
There are not really any variants of them - its just that some of them have some different shades of hair and body but thats probably due to batches and manufacture more than anything else.
Good luck tracking them down! They are adorable