I am sharing with you the link of an article containing information on auxin quantification....You can read, compare and choose the best one that is suitable for your trial. http://www.sciencedirect.com/science/article/pii/S0003267015013148;
I have tried using the phytodetek kit which is also an elisa kit (www.agdia.com) but was not happy with the results. The variability between the wells for the technical replicates was enormous. I suggest using UPLC-ESI method if you have access to such instrumentation facilities. The variability is far less as compared to ELISA.