Use online to find the equipment you want. I'd suggest putting the question on the General Board as to what system folks have and which ones work well at high speed. Also, what will be the input? Satellite radio, mp3, etc. Bluetooth or wired?
Once you have a brand and model, then use the website of that company to locate a local retailer of the equipment so you can go and look at it, discuss with the sales folks, get questions answered.
Here's a set of speakers on Amazon that have a lot of reviews and most are good
https://www.amazon.com/BOSS-MC470B-Motorcycle-Bluetooth-Audio/dp/B00IBI5134Their website shows that their products are sold at Fry's Electronics, Pep Boys, Advance Auto Parts and others.
Good luck!