GE designed and built several models of Elec-Trak electric lawn tractors in the '70s. Mine is a 36 volt model built in 1971 (more pictures below). It has a front-mounted mower deck with casters.
I broke the right hand caster. There are a few people making parts, like the Electric Tractor Store, but I needed to mow the back yard this weekend. Off I went to the farm supply to see what I could piece together.
If you want an electric lawn tractor, your choices are limited.
(1) There are many of these 1970s Elec-Traks still going strong, but people don't seem to like to part with them until they die (either the people or the tractors). They seem to be most common in the Midwest -- you might pick one up there for less than a grand (eBay or Craigslist). Kansas Wind Power sometimes has one. Check out Elec-Trak.com for tons of information on these durable machines!
(2) You can order a new Electric Ox. from Canada. Sweet electronics, no mechanical transmission. You may need a loan though.
(3) You can wait for Modern Electric Tractor Inc. These folks are setting up to build updated versions of the Elec-Trak design, as well as improved models. Word is the prices will be similar to those of professional mowers, not consumer models.
(4) You can mod a consumer gasoline tractor. Later, you will weep as all the expensive parts you added lie among the rusty ruins of the crummy donor tractor.