A further option is Shoe Tags from Timing Sense. We have 500+ and use them for Road, Trail and Cross Country. Reusable so cost effective.
Will cost a few $$ each. You can ask for them to be coded if you wish or you can code them yourself.
I have not tested with US frequency so you may need to check that.
Simple Tags from China - cheap, need to add fixing,
ShoeTags from Spain - a bit more expensive, easy to get on and off
HuTags - More expensive again, probably overkill as they are designed for use in more extreme conditions. ( Triathlon, Mountain Events) Good quality and reliable. last for years.