I've been buying my Piccolo strings from Ken Smith for 15 years now.
It's a Nickle Plated "Burners" 5 string set made up for me.
I'm quite sure if you call him he'll sell you some.
My gauges are:
18-25-35-45-60
An easier way would be to look around online.
Here is an example I found rather quickly.
A 4 string Piccolo set:
http://www.juststrings.com/dad-exl280.html
Add a 5th string as a single:
http://www.juststrings.com/dad-xb065m.html
Easy as that you have a 5 string Piccolo set.
Ideally you should have the nut recut by a good repairman but,
you should try the strings first and see if you want to commit
an instrument to this tuning before spending cash on repairs.
Just lower your action a bit and redo the intonation and give it a whirl!
Piccolo basses are a blast for wanking at home or laying down guitarlike parts on recordings etc.
Good luck,
Rick