To me, the thickness of the mono is more important than the pound test rating. It should not be bigger than the end of the fly line. For most 6 weight fly lines, .024" monofilament works the best.
The industry standard for butt sections is Hal Janssen's Leader Control in 30 pound test (.024" diameter). It is also the same product as clear Amnesia in just different packaging.
After you tie your nail knot, it is recommended that you coat the knot with some sort of sealant. This coating will help keep moisture from wicking into the fly line. At the shop, Rio UV Knot Sealer is used.
---------- Post added at 10:41 PM ---------- Previous post was at 10:37 PM ----------
Or you could weld
your own loops. It's pretty easy.