Here are a few options for external RF amplifiers for use with the FLEX-1500.
Interfacing a FLEX-1500 to an amplifier is pretty quick work. There is a PTT OUT line on the DB-9 connector on the radio that connects to the amp PTT switch.
RM Italy 250W - $650.00
Pros: 250W out, Great Value
Cons: Not FCC certified, No 6m.
Tentec 418 Amplifier - $785.00
Pros: FCC, HF/6M, 100W, solid company
Cons: None really.
Elecraft KXPA-100 - $750/$1120
Pros: FCC Certified, 100W, HF/6m, EXCELLENT company, Available Antenna Tuner
Cons: A little Spendy when you add the ATU
HF Projects HF Packer - $281
Pros: Low cost, optional high pass filter toroid set
Cons: This amp is a kit, 35W output, no 6m
HF Projects SuperPacker Pro V2 - $549
Pros: High quality kit
Cons: This amp is a kit, 100W output, no 6m
HardRock-50 - $299
Pros: Supports PowerSDR band select, high quality kit, through hole assembly
Cons: This amp is a kit, 50W output