Firstly, how do you obtain the entity ID of an item, because I don't see them anywhere on the info pages or anything. I thought it would appear when you go to craft, but I want to get the custom before I start in-order to not retool..
Secondly, if I plan to get public customs are those IDs different from the default? If so how would I get those as well because all the public customs let you do is view them and in turn download them if you were going to upload it again.
If you own the item, the ID can be found in the Inventory right above its name.
If you don't own the item it's a bit more tricky, but you can find the ID by using your browser's "view source" or "inspect element" features.
If you want to use a public custom, you have to put the entity ID into the CP request page and select the "Public" dropdown option. You will then be given a list of public custom images for that entity type to choose from.