As for your question, I just search every available "mainstream" option (eBay, COMC, Tradera (aka "Swedish ebay"), etc) and then I just make sure my wantlists are up to date and I'm online as much as I can to make more people aware that I'm looking for said card(s). Really involving the community around you helps a lot. I don't know what my collection would look like if it wasn't for collectors pointing me in the direction of cards.
I also google search a lot for crd images, hoping it can lead me to an active email address or username. I also search trade lists sometimes to see, by chance, someone has something I need. Usually come up empty since my wantlist is very specific, and that's okay, but sometimes you get lucky and find a card or two.
Also, patience is key. That word may be run unto the ground, but it's so, so true. Keep at it, make your wantlists known, and keep digging and digging until you find it. Sometimes it takes more than a year, especially for those older sets of a highly collectible goalie and soon to be HHOF'r.
But don't fret, and keep at it. The day you find one will be extra sweet because of the time spent searching for it. It's like finding buried treasure.
