Where can I find a Mermaid/fishtail Skirt Sewing pattern?

Simplicity has some that might work. Have you checked there? Go to www.simplicity.com and then click on the link to the Special Occasion patterns. The ones I thought might work (with a little fixing as you sew them, of course) were 4401 and 2251 and 2253. One of them is short, so you'd have to add the flared part at the bottom. The top part is the hardest to shape, though, and you'd have the pattern for that part.