i loathe visiting Sprint stores so i checked if i could order a SIM card online. i was able to do it through chat, but you have to go through the Sprint Worldwide chat page here. if you ask on the regular Sprint chat page, they'll just forward you to the Sprint Worldwide page. you just need to provide them your account PIN and mailing address.
the person I chatted with didn't provide a price for the SIM so i'm assuming it's free. standard shipping is free (3-5 days) and overnight shipping is $12.95.
Update: some people have reported that they were unable to order a SIM through the Worldwide chat page. some have reported that b/c they have a SERO account, they were unable to order. i have a SERO account and i was able to place an order without issue. there seems to be no rhyme or reason for this, and success or failure depends on how knowledgeable of a rep you get. i would suggest you keep trying different avenues of contacting Sprint if you can't order the first time. hopefully, this situation improves as more people place orders
Update 2: for those who were able to place an order and received an email confirmation, you can check the status HERE. the order number provided in the email doesn't have dashes. be sure to put a dash btw the 4th and 5th character and another one btw the 5th and 6th character (like the second example), or else the system won't recognize it