Funny enough, you actually can build a really good quality one for yourself for at least $25 in material (cardstock--> $4, fiberglass resin -->$15, and fiberglass cloth -->$5). THis is of course assuming you have a computer and a printer.
Lucky for you, someone has already gone through the trouble of creating a pepakura design for this. Essentially all you need to do is upload the pepakura program, print out the pieces, put them together (which is easy enough if you're patient), resin, then paint it. Then viola you've got an excellent Spartan helmet. The rest of the armor exists also, you just need to upload them.
The visor however is a bit tricky as this one should be done for last.
So I hope this helped and sorry if this wasn't helpful enough.
When in doubt, just take the next small step.
With that next step take a deep breath. It calms the mind.
When the mind is calm you realize the best is still yet to come.