Yes, you can order Powerwalls from a third party, but you may not receive them for a long time. As I have found out after the fact, some third parties are more special than others and have shorter lead times on Powerwalls due to higher volume deliveries. My two cents would be get an install date with penalties in the contract if you go the third party route.
I ordered Powerwalls in May of 2020, and we are still waiting with no estimate on an install date. New customers from my third party installer are being told late 2022/2023 for installs due to slow deliveries by Tesla.
YMMV...
All the best,
BG