If you have a bit of experience and know what you are looking for buying online is fine. If this is your first time buying a flogger you should use caution online shopping as many large commercial sites stock the same mass produced in foreign places stuff. However, there are plenty of flogger makers here on fet and many of us have our own websites or sell our wares on ETSY of other places.
If you don’t have a lot of experience wielding floggers it can be hard to know what size/length you will be comfortable with, if you can make it out to an event with vendors I would recommend it highly as it can be very helpful to pick up handle and swing a variety of things to figure out what you like.
If you have any specific questions feel free to email me.