Anyone know of an FFL dealer that would sell a stripped AR-15 lower (or assembled lower receiver) to a non-resident in the Palm Beach County / Miami area?

No proof of local address.

US Citizen with Puerto Rican Driver's license.

a stripped lower nor an assembled lower are a "long gun" so i don't think an out of state resident would be able to purchase from an ffl.

Bingo, no proof of FL residency = not able to purchase

Actually, I was thinking about this the other day. If one doesn't have a FL DL/ID, then what does an FFL accept as proof of residency?

Utility bills, voter registration, other?

General Identification Requirements
You MUST verify the identity of each non-licensee
buyer by examining the person’s identification
document(s) prior to transferring a firearm. A proper
“identification document” is:
1. A document containing the name, residence address,
date of birth, and photograph of the person;
2. A document that was made or issued by or under
the authority of the U.S. Government, a State or local
government, or a foreign government;
3. A document that is of a type commonly accepted for
the purpose of identification of individuals.
Common examples of acceptable identification
documents are a valid driver’s license or a valid State
identification card issued in lieu of a driver’s license.
Social security cards are not acceptable because they
do not contain a residence address, date of birth, or
photograph. However, a firearms buyer may be identified
by any combination of documents which together
contain all of the required information (as long as all the
documents are Government issued): name, residence
address, photograph, and date of birth.

There is no reason that I know of for a Florida FFL to not sell a rifle or shotgun to a resident of another state as long as the sale would be legal in the purchaser's home state.  Not sure if territories are included.

Not true.  It is not a long gun (rifle) until a barrel assembly (upper) is attached.

I stand corrected.  Thanks.