Green-Cheeks are an excellent "next step up" bird. I recommend them to anyone looking for a slight challenge and aren't quite into Tiels. It sounds like you've already chosen a species, but I LOVE the whole group of poicephalus parrots: Jardine's, Meyer's, Senegal, and Red Bellied Parrot:
Jardines:
http://www.petbirdpage.com/breed.asp?breed=jardine

Meyers:
http://www.petbirdpage.com/breed.asp?breed=meyers

Senegal:
http://www.petbirdpage.com/breed.asp?breed=senegal

And the less popular, Red Bellied:
http://www.petbirdpage.com/breed.asp?breed=redbelly

You might also want to look into the whole Pionus group:

Maximilian:
http://www.petbirdpage.com/breed.asp?breed=scalface

Blue Headed:
http://www.petbirdpage.com/breed.asp?breed=bhpionus

Bronze Wing:
http://www.petbirdpage.com/breed.asp?breed=bronzwng

Dusky:
http://www.petbirdpage.com/breed.asp?breed=dusky

White Capped:
http://www.petbirdpage.com/breed.asp?breed=wcpionus

If you're looking for a fantastic, quiet companion, I highly recommend you look into the Pionus parrots. They rarely scream and make soft, bleating noises. I would compare their noise level to that of a budgerigar. On the other hand, I am not a fan of the Aratinga conures (Sun, Jenday, Dusky, etc.). These birds tend to scream just (or as little) as much as other species, but their screams are EAR-SPLITTING. I'll tell you one thing. You do not want an aggravated Sun on your shoulder because these guys have high-pitched calls that can deafen you.

As for where you should acquire your bird, the best option is to visit a bird fair/show or find a breeder online. Birds are unique in the sense that it does take a lot of calculation and care to successfully encourage two exotic parrots to mate and it takes even greater care to hand-rear chicks. For these reasons, there is a smaller risk of unreputable breeders, unlike dogs. I would suggest you look on birdbreeders.com and ask the heck out of the breeder. He or she should readily give you a detailed account of what your bird's life will be like prior to the sale. To avoid nervousness and jealousy, the breeder should expose your future bird to a wide variety of animals, humans, and normal noises around the house. If he or she doesn't, run the other way. Socialization during birth up till the point the bird leaves for its new home is a crucial period. Finding a good breeder is very important Good Luck and share lots of pictures!