When you put the a/m bumpers on an Expy with parking sensors, you have to figure out how to mount them. I didn't want to half ass it and I didn't want to waste my money buying sensors and retainers not knowing if they'll work.
Last night I was able lock down what I need to do. It was more than I wanted to spend on the retainers ($38), but I got the information I needed and they'll look awesome. I wanted to use the ones off my original bumper, but I was missing one that I couldn't source and I was going to have to epoxy them on. I didn't think they'd hold in the long run because of the bumper coating. They also might have had issues because they didn't stick out enough. You can't buy them separate from the bumper cover apparently.
I ended up with the following part number. There are actually four different numbers for the four front retainers. FL3Z-15K861-AA (AB, AC & AD)
In the end, these will match my bumper perfectly and, more importantly, will work correctly. The only thing I had to do to my bumper to make them fit was sand the coating out of the edge of the hole in the bumper.
These are what the factory retainers look like: