You might try Lilypons in Adamstown, Md. That's just a few miles from Frederick just off 1-270.

There is a "pond & aquatic plant" forum here @ GW where you can also get information.

Lucky you to have a pond. Have fun.

Here is a link that might be useful: Lilypons

I am not familiar with water garden plants, but I do frequent Behnke nursery. They have a whole section devoted to water gardens in their Beltsville, MD location. They also have locations in Potomac, and Lothian, MD. Good luck!

Here is a link that might be useful: Behnke Nursery

what part of MD are you in?

Plain old goldfish do fine without a filter. Having a water circulating pump helps oxygenate the water, but is not strictly necessary either. Lilypons is one of the best nationally known water garden stores, and a trip there is quite fun, but any of the good garden places like Behnke's will probably have what you need.

I'm not sure exactly where you live, but there is small specialty water garden store in Gettysburg, PA. It's called Gettysburg Water Gardens. It might be worth a trip for you. They have nice sized plants for sale. I used to live in Hanover, PA where we had a pond. We got most of our plants and fish from there.

Behnke's is the best place to go if you live in the area. The filters they sell are well designed and made by Maryland Aquatics.

The following place, "the nation's first wholesale wetland plant nursery", offers approximately 120 species of native wetland plants of the northeastern US. They can ship many or most plants by UPS; some (that are difficult to ship because they need to be in water all the time) must be picked up at their retail outlet.

Environmental Concern, Inc.
201 Boundary Lane
P.O. Box P
St. Michaels, MD 21663
http://www.wetland.org
Tel: 410-745-9620
e-mail: horticulture@wetland.org

Take a look at photos of the following (bet you're tempted):

Asclepias incarnata, Swamp Milkweed
http://plants.usda.gov/java/profile?symbol=ASIN
http://www.ct-botanical-society.org/galleries/asclepiasinca.html

Hibiscus moscheutos, Marsh Hibiscus or Rose Mallow
http://www.botany.wisc.edu/garden/db/speciesdetail.asp?genus=Hibiscus&species=moscheutos

Iris versicolor, Blue Flag Iris
http://www.ct-botanical-society.org/galleries/irisvers.html
http://www.northcreeknurseries.com/Plants/Iris%20versicolor.htm