AK-450
Richard,
Ameriking AK-450 uses Duracell D batteries and is shown to be available for less than $200 on several sites. My understanding is these still meet the requirements to have an ELT. Lots of discussions about the value of 121.5 ELT compared to 406 ELT. Also note that you may not be REQUIRED to have an ELT during your phase I testing since the plane will only have a single occupant.
Regards,