Yup, a GP can prescribe them.
Nizoral AD 1% is OTC and cost around 10$. I got mine at WallyMart for 9$.
Rx 2% Nizoral is around 30$. There's also a pill - that costs around 60$ for 14 tablets. Generics are available and cheaper.
Anti-fungal medications, like the Nizoral pills, have many side effects and can do yucky things to your liver - you should probably only take them if the Rx shampoo doesn't work.
In severe cases ultraviolet light given at the dermdoc's office - like a tanning bed for your scalp
. A friend of mine with really bad psoriasis (all over) swears it's the only thing that's ever really helped.