Yep. So each of those Venusaur/Charizard/Blastoise decks contains random cards. Bulbapedia has lists of what you can possibly receive in each deck. It takes a minimum of 6 decks to obtain each card, assuming you do not receive any duplicates. Pidgey and Pidgeotto are exclusive to the Charizard deck, which is a more difficult deck to find.
All of the cards in the decks are part of a “set.” So they are all numbered ###/052.
There is a youtube user that collects Pidgeys. I have seen him multiple times commenting on videos for trades. I can find him and see if he would be interested in contacting you to swap some cards. I’m sure he has a pretty good ideas about what Pidgeys are out there haha
It’s fun because you need cards from all kinds of sources. Decks, packs, kits, promos, etc. less random than a regular collection and you have a large goal in sight.
I’m so glad more and more people seem to be interested in single-Pokemon collections! It’s awesome to see what you can acquire! I really like your idea of having a Pidgey collection as I feel it’s one of those Pokemon that goes unloved but has some really nice card art. Look forward to seeing you amass your collection!
I’ve been tempted to complete some side collections, but I don’t have the luxury to do so. I kind of wanted to do a Primeape collection after my first Nuzlocke challenge, but again, money.
Glad you’re feeling some pleasure and gung-ho about this!