Hi Noelle!
Congratulations on running your first half marathon! I hope you had a great experience:)
I am sorry to hear you lost your magnet. I have been in the same situation. Generally, I forget about them when going through a car wash, and they often get blown away.
There are a couple of options for you. Occasionally, the online
Disney Store will carry the magnets. I ordered a 26.2 magnet after the marathon in January from there. Right now, they don't have any listed, but you can keep checking back.
Of course, eBay is always a great place to look for things like that. I did see two of the runDisney 13.1 magnets listed on there.
If you Google the phrase "Disney 13.1 car magnet", you will find a couple of unofficial Disney stores that carry them. This search also brought up some cute homemade items in the Etsy store.
I'm not sure how often you get to Walt Disney World, but at Downtown Disney, there is a shop called Team Mickey Athletic Club that carries a lot of runDisney gear and items. The merchandise is always subject to change, but I have seen the magnets there on previous visits.
And, lastly, they will always be for sale at the runDisney race weekend expos. Usually, there is the generic runDisney magnet, plus race specific magnets. If you are running another race, or know someone who is, you can find them there!
I hope this helps! Please let me know if you have any other questions.