There may be an example in the Service connect users guide to point you in the right direction. I think it may use the Service Connect task manager to provide a web page for the user to approve pending POs
If you are looking full blown Requisistion Management then I believe one of Epicors Partners in Australia. Precise Business Solutions, have a bolt on named Epicor Advanced Requisitions Management. I believe it's predecessor was called eReq.
Cheers
Simon Hall