I've heard that some people had good experiences with online purchases. The seller can adjust your HAs on online the computer. I have no idea how they do this.
I used a local audiologist because I can take my HAs in any time and they will fix then while I wait. I also feel more comfortable having a qualified, experienced audiologist monitoring my situation. I can drop by the office ro pick up parts like tubing and ear domes.
It's really a decision about what you're most comfortable with. If you don't mind mailing in HAs and waiting for them to be returned, you can save money. If you don't want to be without your HAs, it's worth spending more.
You might also consider whether the online dealer will take back the HAs if they don't work for you. An local audiologist would be more likely to exchange them with a restocking fee. Ask that question no matter who you decide to use.
Good luck! Hope that you get new HAs that you love!