Wotcha and welcome
Had to check this out with my van, as long as the rims are appropriate for the vehicle, even after market ones are fine, so for example if I were to run 19" car rims with XL tyres on my van and I had an accident I could be in breach of insurance regs because the wheels are not designed for the van - even if they didnt fail. However if (as I have) run a set of TUV approved aftermarket commercial alloys with appropriate tyres I am fine.
So if you run a set of 16" aftermarket steels approved for the vehicle weight and corresponding tyres you should have no problem
or give these chaps a ring
http://www.ebay.co.uk/itm/16-Steel-wheels-Subaru-Forrester-2008-Perfect-for-winter-full-set-of-4-/120895436996?pt=UK_CarParts_Acc_Wheels_tyres_Rims_Car_Wheels_ET&hash=item1c25edf8c4
http://www.ebay.co.uk/itm/Subaru-Forester-15-Inch-Steel-Wheels-/310645623709?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item4853ec0b9d