Once you have a pattern, you'll need fabric. The most beautiful fabrics are very expensive, but there are alternatives to help you get started without wasting tons of money on an experiment. For example, in San Francisco there's a chain of fabric shops called "Discount Fabrics" that sells to the public at way-below-retail prices. And they have sales on top of that! I once was able to get some very plain 100% polyester 2-way stretch crepe in a very pretty shade of light turquoise for only $1.98 a yard. I bought 13 yards, although I usually only use 10 (of 60" wide) for a dress. My fabric cost came to under $30, so if the experiment fails then I've only lost a very small investment. And I can spend some of the money I saved on extra rhinestones! As I get better at grafting patterns together and getting things to fit right, I'll venture spending a little more on fabric.
You basically need to find a couple of good fabric stores and wander in there on a fairly regular basis. If you see something you like at a price you like, buy it and stash it away, even if you're not ready to make something. You never know when a bargain will surface.
It is worth it to experiment if you have the time and interest. When you get started just pick out fabric that is cheap for practicing. It doesn't have to be pretty, because you most likely won't ever wear the first gown you make in public anyway. I'm not sure I see the utility of practicing on muslin if you are making a dress out of stretch material -- it would be better to do it out of stretch material in the first place.
If you're new to working with stretch fabrics, avoid velvet in the beginning. Velvet slides and creeps around when sewing long seams, and it takes a while to to get good at working with it. One of my favorite fabrics is "slinky," a stretchy polyester/acetate blend with faint ribbing. It drapes beautifully, and the ribs make it easier for you to lay out your patterns for cutting.
If you intend to make a bodysuit as part of your dress, it's best to choose a 4-way stretch fabric. This is fabric that stretches across as well as up and down. If your fabric is only two-way stretch, you'll have to add extra length to the bodysuit so that it will fit properly.