Acorn4 - What is available might depend on which country you are in. Some clue as to your whereabouts might bring local knowledge of suppliers.
I have over the years had good results from Vega, D'Addario and various others. Ernie Ball do ones called Earthwood 5 String Banjo Frailing strings which I liked, but haven't seen on sale locally for a while.
In the UK, Eagle Music (already mentioned) sell custom sets and individual strings. Likewise The Music Room at Cleckheaton do individuals and sets (branded as Redwing).

As Don Firth says, (in standard tuning) if you fret the 1st and 5th strings at the same fret at any point between the bridge and 5th string nut, you will get the same note, hence the same gauge string is used.

Quack!
GtD.