Will prowse has a link to the vendor he chose in the youtube video description, if you search for "supplier" in the forum you'll get plenty of feedback on various known vendors.
I chose one (after sending a few private message to forum members) but I'm not recommanding it until I get and test my batteries.
My dad gave me Will Prowse's book as a resource when I was rigging my van with solar - it was a great read but not a necessary buy because all the info you need is on his website.[0] His YouTube channel is a great resource that I'd definitely recommend! Interested to hear what the brand is if your tests turn out satisfactory.
I chose one (after sending a few private message to forum members) but I'm not recommanding it until I get and test my batteries.