If you're saying you have spacers built in to the bells of 15mm, that is perfect to achieve the offset that you want with ET33 wheels. The higher the number, the more INSET the wheels are, so if you have 15mm spacers, then this would make the wheel offset equivalent to ET18 (subtract the spacer width from the wheel ET to get the new ET), which depending on the width of the wheels, is perfectly adequate (unless of course the car is already a wide-track Grp A car, which is what the ET33 wheels were designed for) .

So in answer to your question, another set of 15mm spacers should do the trick, but you will need these front and rear. Eibach do some this thick and Cossymad off the BB can supply them if you can' get them in your country.