Click on the LOAD icon and press and hold both Mouse Buttons for 5 seconds (and then release), a screen will appear offering you a HARDMAN option and Level Section.
Go to the save game option, when the game requests a name type "JOOLS". "Cheat mode active" will flash at the bottom of the screen and the ranks and abilities of your men will have increased. It may be "JOOLSA".
Here is some information on where the Supa Dupa range of weapons can be found. There is only one of each Supa Dupa product, so they're spread out across the game from mission eight onwards.
Supa Dupa Missile Boosta - Mission 8, Phase 3. Go to the bottom right corner of the screen and it's hidden behind a big bush.
Supa Dupa Bullet Proofa Vest - Mission 12, Phase 4. After you've blown up the turrets and boarded the helicopter, fly to the top right corner of the screen. The vest is hidden in the fat snowman.
Supa Dupa Rank Boosta - Mission 16, Phase 2. Top left corner of the screen. It isn't hidden and it looks like a sign for a general (four stars).
Supa Dupa Troopa Boosta - Mission 20, Phase 1. This is a big yellow S in the middle of the screen on quite an easy level. It's still worth getting since it improves your rank until you die and it gives you a Bullet-Proofa Vest, Missile and Rank Boostas for the phase. Watch out when you pick it up though, because there is a spike nearby.
Supa Dupa Troopa Boosta - Mission 22, Phase 2. It's in the bottom right corner of the screen and is guarded by a turret. It's best to blow up the helicopter with grenades to start with, then rocket the turret before getting the Troopa Boosta.