You might find it difficult to get a supplier to deliver to a personal address. You will probably need a business account and a business address. This is certainly true of diving gasses such as oxygen & helium from suppliers auch as BOC and Lindegas,.
Also the rental cost of the cylinders is fairly high, so you need to turn the cylinder round fast to cover this cost. Depends on how many cylinders you need to fill.
I assume you will be decanting into your smaller cylinder(s) and do not have a pump to pump the gas out. This will also mean that you will reach a point where both cylinders ( supply and destination) have the same presure. i.e 50 bar in each cylinder . Unless you have a few spares or are filling cylinders for others, the remaining gas (50 bar) goes to back the supplier when you exchange your supply cylinder.
You coul d keep on partially filling sucessive fills but each time the gas equals out the gas won't flow any more. The gas in your small cylinder will need to be refilled at shorter and shorter intervals and the fill pressure becomes less and less.
Depends on the ratio of the volume of supply to destination cylinder how much gas you finally waste. This may be a lot of gas in a large supply cylinder. It could be equal to severall small cylinder fills, but you cant get it out because there is too small a gas pressure.
It may be a better option to use the larger cylinder to feed the supplies to several tanks via a manifold. I think this is what Tom bar does.
Either way ther will be a final pressure at which the regulator will not work. The larger the cylinder the larger the volume of gas in that cylinder you have to waste.
Try a search on partial pressure filling of gasses and or casscade and decanting methods for gas filling
Hope this makes sense.
Neil