The type C "Europlug" adapter can be obtained from a lot sources - travel shops, Amazon, this website, even ACE Hardware (where it is labeled as "southern Europe") etc.
Actually, the adapter labeled "northern Europe" will work in Spain as well. The difference is the diameter of the two round pins. The northern European adapters have pins that are 4.8 mm (~3.16") diameter, which is the same as the pins in the German Schuko plug, which Spain uses. Italy's plugs use 4.0 mm (~5/32") diameter pins, so the Europlug, in order to be compatible all over the continent, uses the smaller pins, but the Europlug will still work in Schuko receptacles.
Almost all, if not all, USB converters today accept a range of input voltages (100VAC - 240VAC) so no voltage converter is needed, just a plug adapter. (Be sure to call it an adapter. A converter is a bigger, more expensive device that converts the input voltage down to 120V for US appliances.)
Most US "heating devices" (hair dryers, curlers, and straighteners) have what are called polarized plugs - one blade is wider than the other - which, on polarized US power, provides an element of safety demanded by US regulatory agencies (like UL). With one exception, continental European power is not polarized, and the two pin Europlug is not polarizing, so that safety feature will be lost. Do not use appliances with polarized plugs in Europe.
For charging phones and pads, you can also use this European USB voltage converter (charger). It's one piece, so no danger of leaving the adapter in the receptacle when leaving in a hurry.