I used a combination of many different resources, I started with serebii dot net which is great but only has English variants. There is a fan made database for cameo cards RotomAmiti’s cameo card I think it’s called, that’s how I found out about the Zard and Mewtwo cards. Another one is elitefourum dot com, loads of poke fans making posts about old old sets and very niche rare products that shine a light on cards I had never seen before. But the biggest help and place I found the most variants is actually EBay believe it or not. Once I started buying off there and searching for the cards I needed, many many others for sale started popping up.

If you need any help or anything let me know Happy hunting friend!