I think it sounds far too weak to install such a wide panel with just small nails outdoors. There are substantial forces when such a piece wants to move a little. I've never installed hidden outdoors, but isn't there something similar to a floor screw or a thicker nail with a head suitable for driving into the edge?
I wouldn't have nailed such thick boards with 50mm brads. It's often said that the nail should go in 2-2.5 times the length of the material you're fastening, so with the most lenient calculation, you'd need a 66mm long nail and drive it perpendicularly into the panel. Since you're nailing into the groove, it'll be at an angle, which means you need even longer nails.
I bought mine at the regular building supply store, in my case GS-byggvaror in Gullbranna. But it's probably available at all "real" building markets, like Beijer, K-rauta, and Optimera. However, probably not at discount places like Jula, Biltema, and Clas O.