A few choice links and resources...

Installers
Schumacher and Associates offers a full range of premium quality installation and maintenance services. They are based near Doylestown and serve the entire SE PA region.

Fernrock Landscapes, Inc. offers the highest quality and attention to detail in both installations and property maintenance. They generally offer their services within the Lehigh Valley area.
Address: 119 East Berger Street, Emmaus, PA 18049
Phone #: (610) 966-4044
E-mail address: fernrock@ptd.net

Nurseries
Edge of the Woods Native Plant Nursery offers a wide selection of high quality, nursery propagated native plants including many of the "hard-to-find" ones. They are conveniently located here in the Lehigh Valley.

Organizations
Association of Professional Landscape Designers This is the local Pennsylvania/Delaware chapter site where you can find info on this organization including a "why hire a professional" section.