Actually was doing some research on this the other day for a friend and here’s what I found.

Used:

• ASUS ZenScreen (MB16ACE) 15.6” - buy used for around 140$ USD on amazon maybe cheaper on eBay

• ViewSonic VG1655 15.6 Inch - buy used for around 120 USD on amazon maybe cheaper on ebay

New: ASUS ZenScreen (MB166C) 15.6" - currently limited deal of around 100 USD new. Its the best bang for your buck imo