Well, I'd say Sinister Forest is your best bet for Gummis. Shadow Ball you'll just have to cross your fingers and hope to find, usually as a mission reward or in a Lv 1 Dungeon.
Not sure about dungeon machines, but I'll try finding one to check.
for tight belt it is found with a 19% chance of it appearing on floors 1-5 of grand sea. or you could use the code I made from a WM generator and take the lazy way out
Client: Pikachu
Objective: Help Me
Place: Tiny Woods 1F
Reward: Tight Belt
??K? S?F? 4???
QF?8 F4?6 (...)???
Don't be stupid. It's just like putting in a Wondermail you find on the bottom of a mission (Your friend gives you the Wondermail for X mission). It's not really cheating as you still have to toil and suffer to get your Joy Seeds and other rare items (Yeah 3 floors of Tiny Woods really beats the crap out of ya)
Besides if you want a challenge you can make a really hard mission
(Like escort a Caterpie to floor 98 of Western Cave, lol)