Percentile of array (without sort?)

I have a 2560x2048 array of float values that I need the 25% and the 75% percentile values. (5,242,880) as a 1D vector. My first thought was to use a bitonic sort and fetch the value at 25% and 75%. But the Bitonic sort I have is for power of 2 arrays, and I don’t want to go to a larger array with dummies.

This got me thinking that perhaps someone has a way of getting percentile without the overhead of a full sort?