I dont know if they have a place where you live. But there is a place called "airgas". And they are everywhere. Thats where i would get it if i were you.
And it depends where you are how much it costs.
I live in austin texas and it is pretty damn cheap. It costs about $10 a month to rent a 160 liter tank. And it costs $120 to fill it.
Oh and by the way that is the only amount they sell. If you wanna only get like 30L then you have to buy your own tank witch will run you about $500. So i would just rent it.
Also, you will need a 1-2L ln2 thermos wich will cost you around $50 if you get it cheap.
Then you need a cryogenic tube to get it into the thermos... this will run you another $100
then you need "a pump" (it not really a pump i forgot what it is called) to get it into your thermos.