Hi Leon, when selecting a lower receiver, ensure it meets the following criteria:
MIL-SPEC Standards: Look for lower receivers labeled as MIL-SPEC to ensure compatibility with the FRT-15L3™.
No Sear Block: Avoid receivers that include a sear block, as this can prevent proper installation and functionality of the trigger.
.
Here are some brands known for producing compatible lower receivers:
Anderson Manufacturing
Aero Precision
Spike's Tactical
These brands typically offer lower receivers that meet the necessary specifications for the Rare Breed FRT-15L3™ trigger. Always verify the product details before purchasing to ensure compatibility.
.
Stay safe & have a great day!

Spikes tactical lower comes to mind.
The Rare Breed Trigger (FRT-15) is designed to fit standard MIL-SPEC AR-15 lower receivers. To properly house the trigger mechanism, including the "dingus" reset lever, the lower must have an open, unrestricted area behind the trigger, often referred to as an "M16 cut" or "low shelf" lower.

Good evening to you!! You can use just about any milspec lower receiver. Receiver's from reputable manufacturers such as Aero Precision, Ruger (who bought Anderson Manufacturing), Spikes Tactical, Double Star. Hope this helps. Have a good rest of your day.
🇺🇸🇺🇸 'Merica' 🇺🇸🇺🇸

Hi Leon and welcome. The Rare Breed FRT-15 is designed to work with standard MIL-SPEC 5.56/.223 AR-15 lower receivers. To properly hold the trigger without modifications, choose forged lowers with generous interior dimensions, such as Aero Precision (M4E1 or Gen 2), PSA (PSA M4A1/Stealth), or Anderson Manufacturing. Cheers and stay safe!