How to find invoice number from tables

Hi all,

I need to get an invoice number in order to test the sapscript form to print, however I do not know how to get an invoice number from tables, vbrk and vbak.

Please tell me the way to find the required invoice number.

I need to find an invoice number that mathces some criteria,especially print type.

Sorry a little mixed question, but I need your helps.

Thanks.