I use a Premiere Equine's Close Contact Airtechnology Shockproof Wool Saddle Pad with an 18inand it fits great.
Found that although the full-size Lemieux fit well that they can rub hair during coat change.
I have a 17.5 jump and my Eurostar pads look plenty generous enough to put an 18 on. They seem to be out of favour at the moment too so if you can find them they're usually reduced.