How far along are you? If you're siggy is right you're only about 10 weeks, right? I think that's a little too early to worry about fundal height. If it is consistently measuring ahead for several weeks, then it's more likely that you could be expecting twins. I don't think most care providers even start measuring fundal height until around 20 weeks.
I've gotten two 3d u/s (in TN and SC.) Both places required some kind of proof of care.
The first place required you to have proof that you already had a 20 weeks diagnostic u/s-I totally fibbed on this by scanning a picture from my first pregnancy on my computer and using paint to change the date. Then I printed it on photo paper and used that as my "proof."
The second place had a form for your care provider to fill out, but it looked like something I could come up with in word. Your care provider just puts their name, office address, and phone number. Then you take it with when you get the u/s-they didn't actually call the office, so technically you could make up a phone # (or use someone you know so if they called your friend could pretend to be your midwife.)
I think most of those places require it as a way to cover their butts. Those places are really only for "fun"-most of the techs aren't actually properly trained u/s tech so they can't make any type of diagnostic measurements.
If you don't have insurance you should be able to get medicaid. If you make an inital appt with a doctors office they will probably offer you an u/s at that appt, especially if you say you're not sure when you got pregnant.