Unfortunately, the physical 3.1 rom chip must be purchased in order to legally use wb3.1 with a ROM image. Also it won't do you much good if you own an AGA or A1000/500/600 Amiga with 3.1 roms as the ROMS differ for those machines than from the A3000.
Your best bet is to scour e-bay for used 3.1 roms, or check with your local Amiga dealer to see if they have any in stock. Price isn't that terrible at approx. $40 USD.
The illegal route that Paul_Gadd is coaxing you on may not work depending on which Amiga model the rom was grabbed from.
