The best and easy way to get a bulletproof patriot is just by completing all missions of GTA 3 definitive edition just in 1 minute.

Generally, after completing the last mission they receive from Ray (Marked Man) in the GTA 3 game, you will get a bulletproof patriot after finishing that particular mission but if you want to get it very easily then you have to paste the 100% mission complete savefile in your GTA 3 savegame location, you can do this very easily from here, all steps are given to this page – GTA 3 definitive edition – (100% PC Save Game) Once done, you can roam inside the city and after seeing that particular car ( bulletproof patriot) in the road you can take away.