Tim,
Best to err on the 24 volt side than go to 30 v.
However the Xformer you need is actually less than 24 volts because of the action of the smoothing capacitor.

From the figure you give, take a 15 volt Xformer, feed it thru a bridge rectifier and you still get 15v but now DC, now wang this thru a 11,000uf cap to smooth it and you get 21.2 volts which is a product of output volts X 1.414 [ formuales is lifth different to this but this is a good rule of thumb ]

18 volt tranny gets you 25.45
20 volt tranny gets you 28.28

but a 24 volt one will get you 34 volts.

RS do some decent trannies, to get an idea look at 671-9038