copy of Invoice

I have called every department of african bank trying to get a copy of the invoice for our furniture that they financed through furniture citty. I was transferred 6 times to different departments and not one person could assist me. We purchased the goods in December 2012, i am busy with a claim for our bed with King Koil and they require proof of purchase. By law, you should be able to supple me with this as the retention period is 5 years, Why is this so difficult?