The platforms you can buy on are iTunes, Qobuz, 7Digital, Amazon and Stationhead.
For albums on iTunes, you can buy each track individually off of the album. It's not required, but it's much more effective for chart purposes.
Why should I buy each album track separately: One song off of an album contributes 1/10th of the album's sales. For example, since MEGAN:ACT II has 31 tracks and you buy them separate, 31 x 1/10 = 3.1 album units meaning you contribute 31/10ths of an album sale. So, 3.1 album sales instead of 1.
Qobuz and 7Digital: Buy each song individually if you prefer and make sure to download your purchases as soon as possible so you don't forget when tracking ends.
Stationhead: Buy the album or song then download it via app or through your email.
You can buy CDs and vinyls from Megan's webstore. Also, retailers such as Walmart, Target, Barnes & Noble.
Megan's Official Store: Buy physical copies and merch from Megan's official website.