check out Aldi or Lidle, they do a decent size one now and then, as for locating chargers, when the date is up you just take it to the dump, that's what I do anyway and buy another, I usually buy the ones with a little gauge that tell you when to replace, or look on ebay and see what comes up!!
My suggestion would be a 2kg Dry Powder Extinguisher, a Carbon Dioxide (CO2) extinguisher, a Fire Blanket and a plan of how to evacuate fast in case of emergency.
Powder is messy when used but is classed as 'multi-purpose' for a range of different types of fire.
A CO2 extinguisher is best for electrical fires and is possibly the one most likely to be needed so having two different types of extinguishers makes sense.
Having read the above posts I was reminded that I need to get an extinguisher for our MH which was purchased in the summer.
I did a bit of googling and found an 800g dry powder with gauge and a fire blanket (good for cooking fires) from Towsure for about £38.00 for both items. I proceeded to the checkout to find that they would charge me about £20.00 for delivery!
I then tried Halfords who had identical items for the same price and this time delivery was FREE! Guess who I purchased them from?
When it comes to highlands etc.
Some couriers class me that in Aberdeenshire.
Very annoying , but you did the right thing, bought elsewhere. & moved on.
I however always mail them the reason why.
Why they cannot use RM as well is beyond me