I know this post is old but as of today 06/19/2021 you can purchase ammo at the Sig Academy Store in Epping. Purchase is limited to 50 rounds per person.
However they have the ammo in stock almost all of the time. For range ammo the price is I believe $22.85 for 50 rounds. Sure higher than a year ago but actually available and good quality.
I purchased 2 50 round boxes of 9MM range ammo just the other day with my wife.
Hope this helps!