Ace glassware has threaded fittings in some of it's glass pieces.  They cost in the neighborhood of $30-$40 a piece and it's hit or miss finding them on ebay. 

I was unable to find any references to what threads they use, so using our metalworking stuff, I found out that the #7 threaded fitting corresponds to a 1/2"-13 screw thread.  The #11 corresponds to 5/8-11 screw thread.  Don't think I have a tap big enough for the #15 yet.

I also found out via experimentation that the threaded fittings used on some flasks are standard bottle cap threads.  So on occasion I can find a bottle cap that fits some of my glassware.  The bottlecaps that are sold for chromatography bottles fit the glass tubes also.

Teflon rod isn't nearly as expensive as the fittings are.

Salat

The few I've managed to acquire cheap were attached to odd pieces of glassware - like a some sort of lid where the bottom was gone or in mixed lots of lab junk that someone didn't know what they had.  We get some really good deals sometimes from people who don't know what they have.  Once I had a fitting then I could compare it to thread charts and taps to figure out what they were using.  Plumbing is confusing - too many different threads just slightly different.

Salat

Found out just how fun it is to try to tap and die Teflon.  Stuff is slippery!
First fitting I made leaks - will have to get a wider rod and lathe it down for the fitted part.  Sounds like a job for the hubby, I suck at that kind of precision work.

Some of the fittings on chromatography are the same as for bottle caps so you can sometimes find caps that fit.  HDPE is usually ok if it's not going to get too hot.  PP is good too.

Salat