Receiver and battery recommendation for holybro qav250


I am trying to build up a Holybro Qav250 for development.

I am really having a hard time to find the Frsky DR4-ll receiver anywhere.

What is a good receiver that is available and fits the qav250 ?

I am also unsure about the battery. Every document recommends the Lumenier 1300 75. Again can’t find that one on stock.

What is a good battery (taking into account that the qav250 will be used for pixhawk development).