Clearly, the right information about activating this phone with a SIM card from Sprint has not worked its way through the massive sprint communications bureaucracy yet. However, there are people that are having some success. The greatest success in getting a SIM card seems to be by using the International Department who is well versed in SIM cards. If you look around Sprint's website you will find a dedicated phone number for the International Department and there is also a chat option for the International Department. I personally called them last night and I have a sim card that is being shipped to me that is supposed to arrive by the 7th. It was free of charge including shipping. Now, I had some trouble as have many people because if you say that the SIM card is for the Nexus 5 they will indicate that device is not in their system yet and they don't know which SIM card to sell you. However, the consensus on multiple forums on the internet is that there is only one version of a LTE/GSM SIM card of the micro size for Sprint to pick from. The Sprint reps just don't know enough about the phone yet to even know for example if you need GSM or not so that is why they are saying they don't know what SIM card to get you. So, everyone that is successful in ordering one so far has ordered the Sprint model number CZ2102LWR SIM card.
Personally, if it were me, when I call I would not mention the Nexus 5. I would simply say that you need to order a SIM card for your device that you are buying and give them that model number. See if you can get through the transaction without mentioning the Nexus 5. So many people have called for this that they may be catching on and have scription place that will not let you do this now but it is worth a try. I've also read on other forums that those SIM cards are back ordered now but I don't know that personally.