I replace them for customers regularly, people expect them to be expensive but as you've found out it's a simple job. You've just got to find a decent hardware specialist, being local to them I use
Northwest Hardware. No matter what I've walked in with I have always walked out with a replacement. Another example is
www.upvc-hardware.co.uk
The window handles are called espagnolette handles, usually shortened to espag handles and espag locks. If the lock is at 45 degrees to the normal spindle rather than search for a handle for it I would suggest replacing the whole lock mechanism as they usually only cost around £10. Open the window, remove the handle and you can then unscrew the lock mechanism from the edge of the window - you will need to take it out to measure it.
For the stiff one if it is only stiff while trying to lock the window but the mechanism moves freely when the window is open it should just need adjusting. There should be adjusting cams in the window that let you move the lock mechanism in and out on the window. If the lock is stiff all the time then just replace the whole mechanism.