When looking at an item on Sailrite, click on a single item. Scroll down a bit to Description - Documents - Reviews menu. Under documents you can usually find good guides.
I buy my thread from Sailrite and have price checked at some sites as WaWak and The Thread Exchange which are recommended on bag making sites. Comparable thread spools and cones are within a couple of dollars from Sailrite and less color selections in the Tex 70 and Tex 92 sizes.
I wouldn't trust buying thread from Amazon. Thread is the cheapest component of the materials I use, so I'm not going to cheap out on it. You want bonded polyester thread.
For most marine canvas projects Tex 90 / V92 is recommended but you can use V69 if you machine doesn't handle V92.
Most domestic machines can handle up to a Tex 70/ V69 thread, Sailrite Ultrafeeds can handle up to Tex 90/V92 thread and industrial machines like the Sailrite Fabricator can handle up to Tex 135.