Looks like Surplus Center sells the ZTR transaxles for $460-600 per pair (Surplus Center)Yeah your right zero turns are expensive, if it floated I bet it would work well to haul ice fishing gear out on the lake.
Don't need a pair. Left and right drives are independant.
For pontoons I wonder if you could just take some culvert (the stuff that looks like a screw) and weld some ends on it to make it float.
That sounds like a pair to me. A pair of shoes is one right and one left.
Here is a pair of ZTR transaxles.
<img src="http://www.tractorbynet.com/forums/attachment.php?attachmentid=410360"/>
There are three levels of those transaxles (from lowest to highest quality):All the ones I had seen 'till now were one housing. The two separate housings would be better, as you could turn them to have the output shafts inline with the screws. Then just a few sprockets to couple the output to the screws.