I've been looking for a way I could contact Adrienne Rich (E-Mail or other ways) but I couldn't find any.
Do you have any idea where I can find an address, an online or "real" one, (the latter, I imagine, is a bit more
complicated to get..) so that I could send her a letter?
You can usually send letters to a publisher and they will pass them on to authors of books they publish - so if you have a book of her poems or can get one from the library, make a note of the publisher's name and if the address isn't also there you'd be able to find it in various directories, like the Writer's and Artist's Yearbook etc . Send your letter to Adrienne Rich c/o whatever publisher she uses or has used and they should pass it on to her, though they may open and vet it first. It takes a while, but will work if you can't find a website or other contact point.
Adrienne Rich is represented by
The Stephen Barclay Agency
12 Western Avenue
Petaluma · California · 94952
I think they'll forward your mail faster than would her publisher(s).