>What is Comiket?
Also known as the comic market, it's the largest doujin (indie/self published) and fan convention in the world where Japanese people come to buy and share doujin products. This includes games, manga, erotic material, music, and other merch. It usually takes place in the Tokyo Big Sight and comes twice a year. The number represents which event this is meaning there have been nearly 100 Comikets so far. The first Comiket event spawned on December 21st, 1975 and now nearly half a many people attend these events each year.
>Can I shop for some of the products online, and if so, then where?
Consult the buyfag guide (buyfag.moe). Personally I use Melonbooks and Toranoana to buy doujins and music. There's also Booth.pm that some artists will use to sell stuff they won't sell anywhere else like dakimakuras, keychains, mugs or certain music CDs. Some artists will go straight to digital and sell their products on DLsite. You may find some stuff on Amazon.co.jp. Here are some sites with the C95 tag already inserted so you can view what's being sold there.
>Pixiv
https://www.pixiv.net/search.php?s_mode=s_tag_full&word=C95
>Melonbooks
https://www.melonbooks.co.jp/tags/index.php?chara=%E3%82%B3%E3%83%9F%E3%83%83%E3%82%AF%E3%83%9E%E3%83%BC%E3%82%B1%E3%83%83%E3%83%8895
>Toranoana
https://ec.toranoana.jp/tora_r/ec/cot/pages/all/event/comiket/95/main/
>DLsite
https://www.dlsite.com/maniax/fsr/=/ana_flg/all/options/C95/from/icon.work
>Booth.pm
https://booth.pm/ja/events/c95
>What's the best way to buy this stuff if I don't live in Japan?
You can use a proxy service or a forwarding service. A proxy service does all the work for you. All you need to do is send them a link and the price. A forwarding service has you send the product to them so them can send it to you. Therefor you do all the shopping. If you're not confident in what you're doing, then use a proxy service. Though it will cost a little more than the other service. The proxy service I use is WhiteRabbitExpress. Very reliable and good customer service. For forwarding, I use Japamart and BiginJap. Both are pretty reliable as well.
With that out of the way, what are you guys looking forward to?