As Boo said, it’s random.
That being said, we don’t have a dedicated Slowpoke collector that I know of though I’m sure @quuador could help out with a list since it’s missing from his list on his TCG Pokemon Specific List.
If you were looking for cameos as well, where Slowpoke is not the main Pokemon but is featured in other cards, then @JoshsOddCollection 's thread concerning Pokemon Cameos here is the perfect fit.
Look forward to seeing your collection. Slowpoke is featured in some wicked fun cards!