In my experience, it is very difficult to obtain. I live in a country where most of these stuff is very easy to obtain and not that expensive, however Seconal is almost impossible to get and if you somehow find it it's very expensive.
Phentobarb was no challenge to get, and not at all expensive, cost me about $70 USD for 10g
N was difficult although not impossible, the cost of it goes around $15 USD for a bottle of 100ml, however it needs a vets prescription (this is hard to get)
So in conclusion Seconal seems to be out of the question for me.