I have an MBC that I got from Dejon a while back for like $30. Even with the adjustment screw backed almost all the way out, the boost gets into the 25+ psi range. I learned to just modulate the throttle and not go over 20psi.
So I only use this MBC when at the drag strip and use the factory boost controller for the streets. Recently though, the boost is climbing into the 30psi range. I've since gone back to the factory boost controller.

Instead of buying a whole new MBC, I'm assuming I can just replace the spring inside the unit. So my question is, can I get a spring from Ace or Lowe's or something? And if so, do I want a tighter spring or a looser one?

I'm using a small 16g, 3" turbo back exhaust, Punishment O2 housing, and speed density. The turbo housing isn't ported.

Simple answer is yes but it sounds like something else may be the issue. have you checked for any leaks between the boost controller and boost source or wastegate? Does the wastegate have proper range of motion? You may be creeping with the 3" exhaust. So you have no creeping issues with stock boost solenoid installed?

I went through this recently and got a few different springs from a local hardware store called Secore, They were 75 cents each and I had to play with the tension to get what I wanted. it should be in stages but I had to cut mine down and use the 3rd stiffest one they offered to get it up to 25psi. I imagine at ACE they should be a similar price, just make sure the spring diameter will fit the body of your mbc.

There's definitely no leaks. I'm pretty good about keeping things sealed and double checking my work. The wastegate could be a factor. Its the factory unit. I have another factory wastegate from a 14b lying around. Couldn't hurt to switch them out and see what happens.
With the oem boost controller it'll creep up to 15 psi maybe. That won't happen until I'm up to 6k+ rpms though. It generally stays put at 12 or 13 psi.

If the springs are that cheap, I might just buy a handful and spend the afternoon experimenting. I want the boost to level off at 19-21 psi and stay there.

Let us know what you come up with after the spring swaps. I run a Hallman Pro and I've contemplated swapping to a stiffer spring to help with higher boost levels but it may only cause spikes. Sounds like it could be the boost controller on your car though.

Success!

So I spent close to two hours on a back country straight away near my house testing various spring and ball bearing combinations. I picked up four similarly sized springs from Ace that more or less matched what came in the Dejon unit.
I managed to finally get the car to hold at 20 psi all the way to redline using a clipped Ace Hardware spring and two ball bearings, with the adjustment screw about three turns in. The trick was creating a weaker spring than what's been in the Dejon unit.

So nice to just put the pedal to the floor and not modulate! Thanks so much for the help!