Yep they are a pain in the a##e, The only place that I have found is Toyota but they are not that expensive. There are two diferent colours, yellow and grey, I have not tested them for strength but could be that they are different. I just replace yellow with yellow and grey with grey.
I may have some spare floating around if you want them.
There is one thing that I have found, if you loosen the bolt that secures the back bottom of the flitch and mud guard in place it alters the load on the plastic arch and stops it from popping off. I f you slacken it of so that arch looks good and the clips stop popping, work out the gap and put washers onto the bolt to create the required gap that will help.
Hope that makes sense.
Steve