Invoicing systems and Purchase Order systems are nearly identical in design. The only real difference is the direction in which the money changes hands. Thus, you can use the invoicing starter solution and just rename your invoices as Purchase orders if you want. (Or just use it as a model for your own.)
Or you can use this link: http://info.filemaker.com/Us_en_FileMaker_Pro_11_Trial.html
To download a trial copy to a machine that's never had FileMaker 11 installed on it and you can use the trial copy to create a copy of the Purchase Order solution in .FP7 file format. You can then use Open from FileMaker 13's File menu to open this copy and convert it to the .FMP12 format needed for FileMaker 13.