Hello Dis2cruise,
Some people buy everything from amazon and/or eBay. Personally I have never bought anything from amazon so I just cant say anything about their customer service one way or the other. I have bought from eBay many years ago with good results, however you should always check out the seller before you buy from them (online and eBay). As for the products you listed. Any decent TDS/EC meter will run you about $80-$100. But You don't really need one anyway. If you have extra money lying around, that's fine, but otherwise don't bother getting a EC/TDS meter.
The AcuRite weather station products are pretty good. But don't expect them to last forever. I have had mine for over 7 years, and the remote censor doesn't work anymore, and I don't really trust the humidity sensor anymore either. But I placed my remote sensor in outside in extreme heat during summer and that may have caused it to fail early. But the AcuRite products aren't expensive and work well.
As for the general hydroponics pH drops kit, that is the only pH testing product you will need. However the one you linked to comes with 8oz each of pH up and pH down. When that runs out I suggest you get the dry pH adjustors. The dry adjustors are far more cost effective. They each generally run about $10-$12 for a 2.2 lb container.
As for the
grow rocks (clay balls), it doesn't really mater where you get them from. If you don't have a hydroponics shop close by, they may cost a bit to have shipped to you. Just look around for the best price, but don't forget to figure in the shipping cost. I try and find places close to me to have heavy things like that shipped from to lower shipping charges.