Theo, for all classes, most LiPo's are approved by the FRA and subsequently the event organisers.
Team RCC used 5000mAh and 8000mAh "el cheapo" Hobbyking LiPo's in Bullfrog, and never had any organiser objections.
Our feathers and raptors , again, cheap Hobbyking LiPo's, and no problems whatsoever.

Just follow the rules on fuses and battery managment, and you can use most commercial/RC LiPo batteries.