pattern help for newbie Sewing Machines Parts and Sales in Los Angeles

I’m a newbie when it comes to sewing. Learned ages ago but never really did anything with it. Now many years later I’m finding I’m wanting to reteach myself mostly to make costumes for cosplay.

I have a costume in mind I want to do that appears pretty simple. Image 1 Image 2 Image 3
I’ve been searching online for a pattern that’s close to the design but not having a ton of luck.

Where can I look for patterns other than the big pattern companies (ie Simplicity, Vogue, etc.)?

And how difficult is it to modify a pattern?

Thanks for any help.