The simplest method is to go to the Driftveil Drawbridge or Marvellous Bridge and bicycle until you see a shadow. Go to the shadow and you will get any of these items:

Health Wing (HP)
Muscle Wing (ATK)
Resist Wing (DEF)
Genius Wing (SP. ATK)
Clever Wing (SP. DEF)
Swift Wing (SPD)
Pretty Wing

all of these items can be sold to shops for $1500. Also, all the wings other than the Pretty Wing can be used in a similar way as Energy Drinks, in other words, they raise base stats, though not as much as the Drinks.

PS: Occasionally, you may run into a Ducklett (Driftveil Drawbridge) or a Swanna (Marvelous Bridge). These are great ways to train low-levelled pokemon.

Wow, I never realized how much those wings sold for. I never was someone to sell items unless I was in dire need of money.

I'd much rather just go through the undersea ruins to get the rare items there.

I just give em all to my Pokemon.

I didn't realize what these wings really do until I saw this thread. I never used them before, so I'm probably going to sell them when I need to.

Wait, the wings are actually useful for something besides single EV's? .-.

Normally items only sell for minuscule profits, so this is pretty interesting that there is such an easy and renewable way to get money.

Very interesting, going to have to look into this a little more.