You could probably find it on eBay for pretty cheap. Or you could get the NWN Diamond Edition which is only about $30 which includes, NWN, SOU, HOTU and a couple of the Premium Expansion packs that BioWare released all on one DVD. Youc an probably pick that up at the nearest Wal-Mart.
Trust me, Layonara is worth what little expense it is to get that last expansion.