Places to Buy Stuff
I have used SY Fabrics for many projects. They have good prices and their customer service is amazing
Silk Baron has a lot of options for silks and other luxurious fabrics. Real silk is not cheap, but I think this is the best place to get it (in my experience so far).
I love getting my notions from this site! (No, they do not sponsor me or pay me to say this...) They have reasonable prices and quality goods. On top of good stuff, their customer service is wonderful.
Many cosplayers will recommend Arda Wigs, and for good reason. Their prices are reasonable and their wigs look really nice.
Although the website is "corset making supplies," this website has costume making supplies, corset making supplies, and leather craft supplies. I love this shop because their grommets are really nice and their quality is consistent. I have been very impressed with their shipping time!
Sewing Information
Using the right needle is extremely important when working with certain types of fabric. I like this webpage as a guide to what needle to use under what circumstances.
Silk Baron has a lot of options for silks and other luxurious fabrics. Real silk is not cheap, but I think this is the best place to get it (in my experience so far).
Other Creators I Like
These creators have been making content for years and have a wealth of information. Some of them even have books!