Lucky Electronic's Store is one of the recommended ones here: Massive post on where to get lithium-ion batteries everywhere in the world!
Mooch's West Mountain Radio CBA IV has a lot of optimizations to increase its discharge/power handling and absolutely minimize or compensate its voltage drop current rate dependence. Yours has this problem. By seeing the distance between your 5A and 10A ratio curves I infer nearly 40mΩ of cell plus lead paths' resistance, while the actual cold DC IR of 30Q cells is less than 25mΩ. Inaccurate, thus.
Cheers 