La Unua Libro
Where can I buy a copy of "La Unua Libro?" I've looked on Ebay, Amazon, and Barnes and Nobles.
Generally speaking, museums and archives do not sell their collections. With museums, it's considered an ethical guideline to only sell items because you want other items, but not for buildings or salaries. Archives simply don't collect things they don't intend to hold on to.
So, that would be "no."
Since you're in the US, the first place you should look is the retbutilo of Esperanto-USA. Amazon and Ebay is often overpriced or contains literal junk - such as machine translations of public domain texts. (I should really start my own Amazon store or review site.)
There's a book called "DEK LIBROJ" which seems to contain a reprint of La Unua Libro. (*) It's not expensive, and it has lots of other interesting stuff in it too. You should be able to find it based on the above, but I think this is the direct link.
(*) Oh wait, looks like it's just summaries of the book. Still might be worth it if you're interested in this sort of thing.
There are several names La Unua Libro goes by in English. The most common translation into English I've seen sold on Amazon is called "An Attempt Towards an International Language" translated into English by Henry Phillips.
I'd recommend checking out the text online first to see if you like the style of it. It is rather dated, being written almost 130 years ago. And it's public domain, so there's no need to buy a copy unless you just want a physical book.
Wikisource has the full text: https://en.wikisource.org/wiki/An_Attempt_Towards_An_International_Language
Here's a link to an 1898 reprint of Geoghegan's translation.