If you're building more than one, I suggest you try and A one one and a B er so on the other.
This will allow you to try different tunings in the different string sizes.
I kinda use left overs, but my preference is for the heavier strings. I use around a .42 and tune to E or G.
I knew I'd be building a few so the most cost effective deal I found was: 18.95 + 3.07 s/h for a total of 99 feet of wire! The assortment was from .031" at 3 feet (piano size 13) to .047" at 17 feet (piano size 21) and each size in between. Length increases with each size up. This should be enough for 25 or 30 diddley bows! The seller on eBay is showard88
The eBay store is Howard Piano Industries
This link takes you to the item: http://www.ebay.com/itm/110452804420?ssPageName=STRK:MEWNX:IT&_...
So I gave ita try and I am not totally happy with it.I may have to give piano wire a try.