I'd suggest checking out RightStufAnime. They offer a decent range of manga titles and often have good deals. Another option could be Barnes & Noble, which also stocks various manga for online purchase.
First, search for well-known and trusted manga platforms. Make sure they have a secure payment system and a wide selection of manga. Also, read the terms and conditions to understand shipping and return policies.
Websites like eBay and Etsy sometimes have comic sellers. Just make sure to check the seller's reputation and reviews before making a purchase. Also, some comic publishers have their own online stores where you can order directly from them.
DeviantArt is also a good option. It allows you to showcase your work and connect with fellow artists and manga enthusiasts. Plus, it has a community that provides feedback and support.