For springs a little larger than the ones we normally use here (but still very useful to have around) there's this set :
https://www.machinemart.co.uk/p/cht3...ng-collection/
RS sell extension and compression springs here :
https://uk.rs-online.com/web/c/enclo...door-hardware/
Don't ask me why they're under 'door hardware' but they seem to go down to quite small sizes.