calibration fluid reccomendation?

[ReefnQueens]

i need to calibrate a few ph probes, and i wanted to buy the fluid by the bottle instead of the packets. does anyone have any reccomendations? i have 3 double junction ph probes, 2 are neptune and one is bulkreefsupply...also has anyone used oakton, biopharm or atlas scientific? its whats poppin up on amazon