https://www.amazon.com/Gravity-Ritter-Hirsch-Kristen-Cardellini/dp/B07BXZPHSQ
https://www.disneyplus.com/welcome/
https://www.worldcat.org/libraries (your local library)
Hey welcome to the fandom. Glad you love the show. Here's a post GF To-Do List, In order if you or anyone haven't already.
Watch all the shorts
Watch it all again and look for foreshadowing/hints
Watch it again and try to decode the ciphers
Get the Collector’s Edition DVD/Blu-Ray and watch all the special features
Read the Journal 3 hardcover
Read the Gravity Falls: Lost Legends graphic novel
Read the Curse of the Time-Pirate’s Treasure Select-Your-Own-Choose-Venture book
Read through the Alex Hirsch AMA and Bill Cipher AMA Listen to the Gravity Falls Gossiper (podcast) interview with Mr. Mystery
Read Anyway, I’ve Been There by Caleb_Nova and A Return to the Falls by J_COTW (Fan Fic but it’s absolutely amazing)
Check out the artwork of MoringMark and TurquoiseGirl35
Get sucked into the Gravity Falls TV Tropes page for some deeper analyses of the series and the tropes it uses (and subverts)