Pre-form
I haven't been able to find any large (150g) pre-form ponds in this
area and would like something around 500g or larger. I've seen that
these are available elsewhere and I'm looking on the internet.
Anybody got any suggestions?

I'm not sure where you are, but Sam's Clubs in the Dayton, Ohio area have a 600
gallon pre-form pond for $105. Their display ponds are set into a wooden frame
covered in something like astroturf. Good luck!