There's dozens of websites where you can read it online for free.

But, if you want a physical copy you can buy it on Barnes and Noble or Amazon

Note, there are two manga volumes for Phantom Bullet.

Here, I'll help you add to your collection!

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SAO Progressive vol. 1 (Light Novel)

https://www.amazon.com/Sword-Art-Online-Progressive-light/dp/0316259365/ref=sr_1_3?ie=UTF8&qid=1542854838&sr=8-3&keywords=sword+art+online+progressive

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SAO Progressive vol. 2 (Light Novel)

https://www.amazon.com/Sword-Art-Online-Progressive-light/dp/0316342173/ref=sr_1_5?ie=UTF8&qid=1542854838&sr=8-5&keywords=sword+art+online+progressive

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SAO Progressive vol. 3 (Light Novel)

https://www.amazon.com/Sword-Art-Online-Progressive-light/dp/031634883X/ref=sr_1_7?ie=UTF8&qid=1542854838&sr=8-7&keywords=sword+art+online+progressive

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SAO Progressive vol. 4 (Light Novel)

https://www.amazon.com/Sword-Art-Online-Progressive-light/dp/0316545422/ref=sr_1_1?ie=UTF8&qid=1542854929&sr=8-1&keywords=sword+art+online+progressive+4

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SAO Progressive vol. 5 (Light Novel) - JUST RELEASED

https://www.amazon.com/Sword-Online-Progressive-light-novel/dp/1975328140/ref=sr_1_1?ie=UTF8&qid=1542854838&sr=8-1&keywords=sword+art+online+progressive

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SAO Progressive vol. 6 (Light Novel) - PREORDER

https://www.amazon.com/Sword-Online-Progressive-light-novel/dp/1975383338/ref=sr_1_7?ie=UTF8&qid=1542854935&sr=8-7&keywords=sword+art+online+progressive+5

Volume 15

Same place you get any other book, Barnes and Noble, Amazon, etc Sword Art Online, Vol. 15 (light novel) https://www.amazon.com/dp/0316390496/ref=cm_sw_r_cp_api_i_43VPCbE3KQRVM

http://www.amazon.com/dp/0316259373
http://www.amazon.com/dp/0316383775
http://www.amazon.com/dp/0316348759
http://www.amazon.com/dp/031631465X

Here you go. There's the whole "(manga)" part in the name, so they are pretty obvious, though.

Good Smile Sword Art Online Alternative Gun Gale Online: Llenn Nendoroid Action Figure https://www.amazon.com/dp/B07F4D68S2/ref=cm_sw_r_cp_apa_i_8a5iDbRVGE8BD

joshuashanevis has answered your question if you were looking for the anime's official OST, but if you're looking for an official music score book, here's an Amazon link for it. There's not much song variety I'm afraid other than a handful of handpicked songs, but if you really want to be specific the only way you can find other scores is from fanmade ones.

https://www.amazon.com/Puzzlebox-Puzlebox-Orbit-Mobile-Edition/dp/B00BTK2SRS

If you can get it off Amazon I'll bet the military has a bigger version already.

https://www.amazon.co.uk/Good-Smile-Alternative-Nendoroid-GSCSAG90591/dp/B07F4D68S2/ref=mp_s_a_1_1_sspa?keywords=llenn+figure&qid=1565019458&s=gateway&sprefix=llenn+&sr=8-1-spons&psc=1

And now your collection can be complete, http://www.amazon.com/Dream2reality-Cosplay-online-Lambent-Medium/dp/B00C7AAQCS/ref=pd_sim_sg_4?ie=UTF8&refRID=0J7XC90YGR8CNSPGZ3EP

Source: http://studiosdt.jp/archives/566

More of his SAO related stuff can be found here: http://studiosdt.jp/gallery (Artist: Yuki Tatsuya)

Some of his work is also featured in the Sword Art Online Comic Anthology 1 and 2 (along with various other artists)

They're not online, but you can buy them here: http://www.amazon.fr/dp/2373020009/ref=asc_df_237302000925124042/?tag=anon-new-21&linkCode=xm2

Also, RealFireNSteel Sword Art Online - Kirito and Eugeo's Blue Rose Sword Turquoise https://www.amazon.com/dp/B07M6BLB3C/ref=cm_sw_r_cp_apa_i_gbE3Db07P9N0P

Amazon pre-order link

https://www.amazon.com/Sword-Art-Online-Progressive-light/dp/031634883X

$7

Can you order from amazon.ca?

SAO 2 Progressive Manga

Edit: Hmm actually it seems do be doing the same thing on the .ca website. Wasn't like this before. Wonder whats up.