What we need when buy a costumized progr

My company had a costumized program made in visual basic using SQL as data base in which we have all rights. It has been almost a year, since x software company began. From the begining there have never been blue prints on the development of this software, so for this reason there have been many problems with this software company that has not formaly deliver our software. What would be a list of things, tools or others this company should deliver to us? Or what should be required to have in our hands in order to sign a new contract with a different software company or individual programmer to avoid any delays on the fixes and modifications that our software needs?