That brings up a resource I forgot to mention,
other pond keepers!
There are probably at least one pond or koi club
in your area. Also watch those tabloid sized ad
papers that come out. Often ponders will sell
divides in those.

And pond plants grow like crazy. The plant you
buy this year you can probably divide into four
plants next year.

As far as planting them - and realize I am still a
total amateur at this (and years don't help...)
but most plants come with a tag that will tell you
how far underwater they should go. Marginal plants
like reeds and sedges and other 'stand up' plants
can usually go from an inch to five inches underwater
at their crowns and they aren't that fussy.
You can use plain garden soil to plant in, they sell nice
rigid mesh black baskets to put the plants in and special
pond tab fertilizer (or use rose spikes). Lilies go on the
bottom.

You just have goldfish right? Koi can cause some problems
but many of them can be overcome with a few tricks.

kathy