AES sells some vintage-looking varnished spaghetti. They have two sizes: #20 AWG and #11 AWG. The 11 AWG is too big. Both are a little pricey: $0.50/ft for the #20, and $0.60/ft for the #11.
My choice is teflon tubing. For example, see: http://www.action-electronics.com/teflontube.htm This stuff is about half the cost of AES' spaghetti. Mouser also sells teflon tubing.
There are two colors of teflon tubing: natural and black. "Natural" is essentially clear. It doesn't look right, so I go with black. Either way, the teflon tubing has a shiney appearance which isn't quite right, but good enough for me.
What size to get? Almost all modern components, resistors and caps, will have #22 or #20 leads. So #20 will usually fit, but often a little snug. My preference is #18, which might be slightly loose, but that was typical for old spaghetti.
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The modern teflon spagetti can be "toned down" a bit by sprayuing it wit satin Krylon. this produces an appearance very close to that of the old varnished cambric tubing. By the way, what ever would one use the #20 spagetti for? It will not cover #12 square bus wire, and I cannot imagine why anyone would waste their time fooling with those modern AC sets! ;)