Where can I price and buy HDPE for geothermal ground loop? What size  diameter. and length for each loop?  I want to run  one solid pipe (no fusion joints).  I will be digging my own trench.  Is there an advantage to having additional loops?  for example 4 loops on a 3 ton unit??  

Might be best to pay someone for a little bit of their time to do the design.

we sell HDPE by the pallet only,
You will need some design assistance on loop length.
Perhaps the person providing the Geo Unit can assist,
One solid long pipe will overburden your pump, better to have several 500 foot runs and a manifold.
If you do not have fusion tools or access to you can use 3/4 Pex B as an alternative.
Dan