I've certainly bought packets of blades on ebay in small quantities of 20 or so at a time, but this seems to be no longer an option. It looks like you can buy a handle bundled with replacement blades but a very limited selection..
Pretty sure I've got some shape 10 blades to fit your number 3 handle you can have, IIRC they come in a little silver foil packet of 5 but won't be sterile. I've got other shapes as well so it's as well MichaelG has found the encyclopedia of SwannMorton.
I doubt if I've got anything to fit a number 4 handle but I will check in the morning. Maybe you can recognise the shape of blade you need from the SwannMorton catalogue. If you would like to advise what shape you want if I do have some you're welcome.
If you're interested drop me a PM.
Rgds Simon
edit – correction I think you'll find that shape blade in the number 3 handle is a 10A
Edited By Simon Williams 3 on 03/11/2021 21:58:40