Talk to Mercury. I lived 2 blocks from them for years, and know that some of the world's best amp techs even work there.
They have a list of techs you could send or take it to. If you're handy with a soldering iron, you may be able to use their diagram.
Besides just a transformer, some of them can make your amp a monster. Check www.krusekontrol.com He does great mods on 5150's and all other amps. The best in the business, and he knows Mercury Magnetics.
Click on his Peavey Mods. You may even forget the transformer when you see all his other options.