Modelling
Stuff of Legends Has images from Citadel catalogs from the 1990s and 2000s. Excellent resource for checking old out of production Warhammer / 40,000 models.
Lore and Fiction
Along with many other people, I find listening to 40k lore videos or audio books is a good way to make modelling and painting more fun. While I wouldn’t normally sit and listen to lore videos (I did do this for the Seige of Vraks series though), having it playing in the background allows me to soak up lore, enriching my 40k experience. Here are some really good YouTubers I recommend (and financially support), in no particular order. If you like their content, and have a little spare cash (I know, spare cash and Games Workshop hobby can be something of an oxymoron), please consider sending them a couple of bucks; we all like the content so let’s support content creators to do their thing.
A Border Prince Superb story teller, narrations of Black Library fiction is a favourite, and good lore as well.
Baldermort’s Guide to Warhammer Our faithful servant, a magnificent narrator, wonderful mix of lore and original fiction to bring the lore to life.
Arch His lore videos got me back into 40k after a 20 year absence, his Siege of Vraks series is magisterial.
Luetin09 Lore and general hobby commentary, his lore epistemology (ie methodology and empirical approach) is to be much respected.