They aren't unobtainium but depending on condition and the quality of the build, such as high grade wood, engraving, sights and of course chambering, they won't be cheap.
I have a couple, one is a lovely little rifle with great wood, special profile, egg shape barrel with a flat, serrated top and plain sights.
It's just marked "380" which also just happens to be the bottom of the groove diameter and the designation is referring to the diameter of the bullet needed, not the cartridge.
I suspect they were built with a lot of leeway, concerning cartridge cases.
This rifle will accept the 380 Webly case with the .380 diameter heeled bullet and shoot it quite well.
I have another chambered in 357 Maximum but it's on a large action, not a "cadet" action.
There is also a nice little Cadet action in 22rf, that I recently put a barrel onto.
Stocks are available from out of country sources as are parts for some models.
Quite honestly, unless you will be mostly shooting indoors or at short ranges, under 50 yards, you will always be looking for a bit more.
That's why I built the 357 Maxi. It can be loaded down or hot. Very impressive cartridge with bullets from 125grain up to 250 grain.
It will accept and shoot 38 S&W, 38 special, 357 magnum as well as 357 maximum well. Very versatile cartridge.
I started a 218 Bee last summer but hunting season came up and it's waiting for some warmer weather for me to venture into the shop.
I kind of shut down when it's dark by 6pm. The days are getting longer, sooooooooooooo.
The rifles in 303 British are great, they can be found with both Metford and Enfield rifling, so if you find one, make sure you know the difference.
These actions are strong, I also have one chambered for the 7x64R and load it to 280 Rem velocities.
If you come across a large action, chambered for the 577/450, chamber inserts are available for 45 Colt and 45acp. A little bit more hassle but still fun to shoot and much cheaper.