Better price at Right Stuf. See reply below.

[Amazon US] Mobile Suit Gundam Thunderbolt December Sky Blu-ray ($19.99) - Previous low was $23, also on Amazon.

They're movies. You can find them on Amazon or at local niche shops where someone sold their collection, or where the owners of a hobby store have a love for all things gundam.

December Sky (first movie) and Bandit Flower (second movie) the third is yet to be released as of my current knowledge. But I don't keep up with it.