I know there are a lot of swipe type apps in the AppStore, both free and paid. However, I don't use them so my only suggestion is to read the reviews and see which ones will suit you.
I just want to add a caution: no app will replace the default keyboard. So, you can use the swipe app to create your text, but you'll have to copy the resulting text to another app. Some may allow you to send emails from within the swipe app, but not all.
Just so you know that you can't replace the stock keyboard...
Marilyn