As mentioned by @nauticads , unlimited edition E1 are very rare. We still don’t know for sure how all of them are released, but most come from a Random deck of which only 50 were supposedly given at an event back in the day. The Pichu and Charizard have the most copies emerged thus far, so they probably had some kind of special release I assume. Since they’re Rares, it wasn’t possible to get them in the random deck. (The Ponyta is the only easy one to find, since it came as a magazine promo.)
Some more info can be found in this thread. Here the amount of unlimited edition copies that have been seen on pictures thus far (mostly in Mercari/Yahoo auctions):
The unlimited edition E1 Pikachu is also one of the 16 missing cards in my Pikachu collection. 
Greetz,
Quuador