So that hearing aids can not be sold on the Internet, it is generally recommended to the professional fitting center to go to the fitting.
The reasoning is simple: if you don't go through a formal fitting process, it's easy to buy a hearing aid that doesn't suit your situation.
The hearing aids that you buy online may either over-amplify the sound, making it very loud and harsh, or under-amplify the sound, making it hard to hear when you wear the hearing aids. It's not worth it if you make a mess out of something that's supposed to be good.