Rhyno,

The "Seima" side marker lights are used on most US Fiat's. I know that the X1/9's are identical to those used on the US Jalpa's. I would try www.baylessfiat.com or www.fiatparts.com or www.vickauto.com. International Auto www.international-auto.com also carries a remanufactured type, but they are not stamped "Seima" although they are identical in every way.

Finally you can try ebay or go through the Eurospares but expect to spend a little bit more for "official lamborghini parts"...
Hope this helps