CristinaGrancila Hey there!
Indeed, suppliers almost never provide invoices and that can cause quite some inconveniences.
However, there is a way around - most merchants use apps or chrome extensions that can generate invoices for you.
You can find some great apps on Google and in Shopify app store.
Here's the link to Shopify app store: https://apps.shopify.com/search/query?utf8=%E2%9C%93&q=invoices
I'm sure you can find other great apps or extensions that can do the job for you!
Best of luck