You can buy the atoms from Sigma Aldrich or any other chemical supplier. You have to buy a lot of atoms, and you have to buy them by the liter.

If you want rocket fuel from those chemicals, you are going to have to pressurize them and cool them or warm them, and you need a lot of energy and equipment to do it. That is the expensive part. Often, rocket fuels are heavily pressurized orsupercooled, and supercooling a gas is expensive. That allows you to store more energy in a given volume.

Rather than digging in after your initial mistake, it'd be better to google it. This is a no-stakes random internet argument, so you've gotten the lesson cheap; but if you repeat the same mistake elsewhere you could do yourself a lot of harm.