You need to make sure that you first have an account on the Blender Market, if you don't have an account yet head to the signup page and create a new account. Once you have an account, you'll need to log in and then head to your Account Page.
Your Order History tab should show up on your Account Page. This is were you'll find links to your purchases and your Receipts/Invoices .
If you need certain specific information on your receipts like a VAT# or company name you need to set that information on your Account Information tab. The Name, Company Name, Address, Country, and Tax ID/VAT # is the info you need to make sure is filled out.
If for some reason your purchases aren't on your Order History form you'll need to contact support at [email protected] because we'll need to link your purchase to your account.