The small gas bottles can be a bit of minefield.
Some use larger bottles at lower pressures, and some use smaller bottles at higher pressures.
Have a search on ebay, as I was surprised at how many suppliers there are when I was considering cancelling the contract for the full size bottle.


As for insurance, having a bottle of LPG in your garage is far more high risk, than a bottle of inert gas that in the worst case scenario might blow the top of the bottle. It's oxy-actylene that insurance companies don't like. One bottle that will go bang and take of skywards with no warning if you do something wrong, and one that'll sit gradually heating up until it ruptures and flatten everything within quite a large radius if you get it wrong, it's pretty easy to understand why insurance companies don't like them.