Does anyone know where to find/buy tuned pipes for rc plane engines(46 size)?

That's weird, I already posted an answer to this question (as did somebody else) earlier today, but now it's gone. I suggest you investigate the following links, and maybe you'll find something.

http://www.keleo-creations.com/

http://www3.towerhobbies.com/cgi-bin/wti0093p?&C=GMA

http://h1071118.hobbyshopnow.com/ser...nCategory=EPMA

Someone (I forget who) suggested a link that was more dead on (tunedpipes something).

bought mine from the lhs its a macs header and tuned pipe for a OS 46fx. the tuned pipe header will work for AX fx and sf 46's. what engine do you have that might make it easier to help you.

Two sources:
Just Engines (in the UK) http://www.justengines.unseen.org
MACS Products (California) http://www.macspro.com
I'm sure there are others but these are the ones I can think of.

True quarter wave pipes with the full internal tract to enable the magic (these are not the empty can mufflers like the popular Tower muffler) you will realize major performance gains as well as arguably improved throttle response. I use a lot of them on various twenty fives, Forty sixes and sixtyones.

http://www.pspec.com/results.asp?category=8

John