I buy almost all of my tri fold cards online but some of them are also photo frame cards. The photo frame cards I purchase at www.studiostyle.com. I just usually do a search of "needlework cards" and shop from there. You can also find them on e-bay. Most of my cards are either oval or have a rectangular opening.
I am so happy to see you posted those photos...I just started making cards. Where did you find cards with square openings??! Mine are all oval but I had wanted squares. Your carda are lovely!!!