You can buy practice rounds for the 106mm recoilless rifle on line today - now the internet in 1997 is not what it is today but it did exist and thus they would have been able to buy the practice rounds which is what they gave the pirates on Grenada. Actual working weapons are harder but if you have the right people you can take a demilitarized weapon and make it able to fire again.
Same with inert rounds - i.e. you need the right skill set which I bet they found and the right materials - but if you know what you are doing they can be used.
And they may have had contacts with the local National Guard to be able to get working 106mm weapons and ammo for them.
FYI one thing people forget is how many weapons are in circulation in this country that you dont find anywhere else - like the fact that there are over 180,000 fully functional machine guns of various makes that are registered in the US.
Also an interesting table that shows weapons unaccounted for by the Marines in the last few years - you could see that happening a lot to account for where NA got its weapons.
http://www.smallarmssurvey.org/filea...apter-8-EN.pdf