Under "Material Eléctrico > Transformadores y Accesorios", search for "Step Down" and it will refine it for you.
If Technical Spanish proves to be a bit awkward to help you find what you are looking for, then try the UK website first http://uk.farnell.com find the item you are looking for, take a note of the Farnell order number and then turn to Spanish Farnell site and look for the same code, and order. Voila!
PS: It's a step-down transformer you want to buy, not a step-up one. You need to step down from 230/240 to 115/120 for your US gear.