i know this is not the best answer
but i got my human hair extentions from ebay and they are AMAZING!!

I like airlocks but they can be a bit expensive and Im pretty sure you have to be licensed to purchase direct from their website. Are you looking for something especially for prom? or more permanent that will last you longer than one night? If you are going for a more temporary option I would consider the clip ins. They are usually a lot more reasonable as the quality of the hair is not the absolute best on the market. These will add length instantly and quickly and will withhold throughout the night. They can also be hidden easily if you are going to be wearing your hair up. You may also want to try your local beauty supply or wig shop as it may be beneficial to see and feel the quality of the hair before spending.

I would try clip ons at first. Go to your local beauty supply and see the different hair they have. Of course human hair is the best and goes up in price the longer you go.