Sheldon did know that lesbians existed -- she was in the Women's Army Corps during WWII, for instance -- but because of her upbringing/social class, the whole world she would have had to enter to learn about strap ons and the like, would have been beyond the pale, unthinkable for her to enter.

Then her upbringing enters the question again, because she was very fiercely taught not to want things for herself. To admit to, to own one's own desires, was something she wasn't supposed to do... which I think was another part of why she didn't just go out and find herself a girlfriend/become a passing woman/buy a strap on.

Between not having a great deal of access to/knowledge of the world of lesbians, not being able to contemplate the huge step down in social class and step beyond the pale in acceptability that entering that world would have entailed, and not being very good at/trained against ever reaching for and taking the things she wanted from life, she was stuck.