Too many to name tbh. I’ve got loads of cards either directly or indirectly thanks to E4 members, some all the way back when I just started collecting. E.g. @woolsluk mentioned noodles (inactive these days) was the dad of the Japanese Art Academy red raincoat Pikachu winner and was selling them back in August 2016, when I was still a noob and very new here on the forum (I joined in Feb. 2016), which helped me get my first Art Academy card at the time:
I’m also forever grateful for @chibicollectables for contacting me first to sell me the last English Pikachu I was missing, another Art Academy Pikachu (the kabuki one by Ashley), when he upgraded his own copy from a BGS-8 to BGS-9:
And although it’s not tied to a specific item per se, I’m also very grateful to the handful of E4 members who link me Pikachu, Seviper, and Yu-Gi-Oh listings on a semi-regular basis and keeping an eye out for anything from my wanted list, even if I do occasionally already own most of the ones that get linked 
, like you @joponnes, @queendrifloon, @bbobrob, @goonhoon, etc. etc. (Sorry if I haven’t mentioned anyone explicitly, I appreciate all of you who help me and encourage me to complete my collection goals!)
Greetz,
Quuador