Really just listen to the music to the three Dollars westerns and you'll get it. Strummed acoustic Guitars and heavy reverb electric guitars for lead lines, harmonica, jaw harp, tack piano, wooden flute, etc.
Deep pounding bass percussion like a death march. Snare flams. Triangles. Spanish corrida trumpet for the ultimate battle. Unusual percussion like church bells, rattles and whips. Vocals used as percussion: Red Room Audio's free "Snaps Claps Stomps & Shouts" would be great, and it has a slap that might work as a Morricone whip sound. Balkan Ethnic Orchestra has great male vocal percussion.
For the soaring "Once Upon a Time in the West" solo female vocal use something with a good legato like Organic Samples Solo Opera or Ethera. Somebody else can recommend a good male choir for the Ohhh-oohh-ohhhh, etc.
You can do something spare with just a few instruments and you'll get the sound, but what makes Morricone so great is the epic orchestral that he puts on top after he has established the basic riffs, and the inimitable choices like the main vocal line on "The Good, the Bad, and the Ugly.
If I was going to do music like this today, with all the tools we have now, I wouldn't just ape what Morricone did, I'd open up to his way of thinking. I'd be looking at libraries like Tarilonte's and EthnoWorld and World Suite to find unexpected textures. There's no law that says it has to be a guitar.