Many of these are UK specific. If ordering from outside the UK, bear in mind there are several planned Royal Mail strikes (with knock on effects on other couriers) so order early as your stuff will probably take longer to get out the UK. I have personal experience with all of these sellers.

Homeware, clothing and accessories

Bookishly
Literary Emporium

Clothing and accessories only

Popsy
Joanie
Rooby Lane

Posters

Spineless Classics

Books and giftcards

Your/their local independent bookshop
National book tokens (UK only)
Bookshop.org
Waterstones at least pay some UK tax

Subscriptions

Graphic Novel Station
The Petersfield Bookshop patreon
Their favourite author's patreon

Charity donations

Your/their local library
The Reading Agency
Book Trust