As most of us are saving for our own tails it may be hard finding anyone here with money to donate. You'd probably have better luck fundraising outside the mer community. If you are looking to go professional, see if you can get any assistance for small or independent businesses. Maybe look into the art community as well. Also, it's important to tell people exactly what you will be doing with this tail and how it's a job. Market yourself! Why should they support you? Do they get anything out of it? Maybe you could team up with a photographer and send people who donate photos of you in your new tail, swimming and working with kids.