My builder uses Houzz Pro to plan the timeline and give detailed estimates and daily updates with photos of progress, and provide invoices at pre-agreed key points in the build. This made the whole experience much more efficient and stress-free.
Pete Nelson is one of the leading treehouse designers in the country (world) and his company supplies all the hardware you could need for a treehouse. Here is the link. http://www.nelsontreehouseandsupply.com
Missy Brown Design