The choice of where to buy Nitrous oxide will entirely depend upon what you are to use it for. For the purpose of culinary whipped cream then you need to buy tiny quantities of approximately 8gm or 2 litres of gas (atmospheric pressure) whereas for use with combustion engines you are more likely to need quantities in he realm of 500g.
Both types can be bought online – indeed they are often cheaper online than in any shop. There are also Nitrous oxidiser compounds that are dissolved in a liquid for adding straight to petrol but these are nowhere near as effective – although they are simpler and can be poured into a petrol tank. You can find these at places like Halfords where they stock the NOS brand. These are liquids fuel additives that boost performance and also contain lubricants and fuel cleaners. This additive does not actually contain any Nitrous Oxide – but claims to boost performance in a similar way.
The N2O for fuel oxidisers requires an entire kit in order to work successfully – so it is really worth asking some mechanic before embarking upon any project – the actual gas is only a small part of what is required.
Buying gas for making whipped cream is by far the easiest with simple cream whipping system costing little more than £20 with everything you need to get started in creating your deserts and whips.
And as ever remember before you buy N2O that it is a significant contributor to global warming and as such – should be treated with a great deal of respect.