I am not sure if anyone just sells the metatarsel guards for shoes. The only thing I have ever seen is Gerry Davis Sports sells a plate/base shoe combo that has the removeable guard you are looking for.
Seeing how that they sell the combo shoe, you may try emailing them or calling them to see what they say about getting just the guard you are looking for. Their website address is
http://www.gerrydavis.com.....Good Luck with your search.