Need to use my 220v preamp in 240v.

How DIY do you feel? For sizing, double the current draw of the preamp.

http://www.signaltransformer.com/sites/all/pdf/DU-SU.pdf

Don't know of any encapsulated transformers for this application.

The preamp is fine at 50 Hz or 60 Hz. A transformer has to be rated for that.

Another detail ...
http://www.hammondmfg.com/dwg14d.htm

And if you want to get creative and be the first on the block ...
http://www.geocities.ws/jonrisch/catch2.htm