Reading the TCS batteries rules AND the Euro Truck rules, if we are using a rounded shaped lipo battery it DOES NOT need to be ROAR approved. I personally have used the following rounded hard-case lipo batteries...Reedy Wolfpack, Peak Racing Power Powermax, & Pro-Tek. They have all performed similarly/very similar lap times in my trucks at the track I race at.
An unmodified TT-01 E chassis requires the used of rounded battery case to fit properly. Therefore any round hard-case lipo battery should be legal from truck racing using TCS rules. Most of the lipo batteries like this online are going to be 4000mah (or less). The biggest MAH battery I've ever found was the Pro-Tek 5000mah, but I haven't found that it offered any significant advantage (at the small indoor track I race at).
I believe Gens Ace also makes a rounded hard-case lipo battery