well, there are a couple of options you have... mostly it depends on how much money you would spend... (not saying that the more expensive transformer and the better ones, just a bit different!)
you could got for george's transformer, they are made by heyboer (
http://www.heyboertransformers.com/index.shtml ). he offers two versions, a "standard" jtm45 (which are shipped with his kits) and a radiospares deluxe ot. the radiospares ot's were used in the very early jtm's. you can find them here:
http://store.metroamp.com/index.php?cPa ... 8ca7ui7ad6
the second option you have is marstran (
www.marstran.com ). scroll down the page a bit and you can see what he's having in stock currently. these are also said to be great transformer, i never heard an amp with a marstran ot, but there are some people here who have built amps with those trannies, so i'm sure you can get some more info about that.
the third is mercury magnetics (
www.mercurymagnetics.com ). these are also great transformer, and the one i use. these are said to be a bit brighter, but that's what i wanted in my 50W plexi... they are also a bit more expensive that the other ones, but i wouldn't rate them better, just a bit different. if i would build a jtm, i probably would go with heyboer or marstran.
you can also find the mercury's in george's shop...
http://store.metroamp.com/index.php?cPa ... 8ca7ui7ad6
maybe there are some more companies who offer transformer, but i don't really know them, and i guess those 3 companies above are the most common options, and either way you choose, they all 3 are big improvements for your amp! same for the chokes...
cheers
rickey