It's a shame that you didn't post this type of question before you bought because I could have helped you immensley. There's nothing out there that prevents you from hiring a home inspector during the building process. Also, there's nothing out there that prevents you from having a home inspection AFTER a new home is built either.
I'm sure you now know that you're gonna be spending 10's of thousands of dollars, perhaps 100's of thousands of dollars on a purchase, and you could have hired an inspector for $3-400 to do a home inspection. That's pennies when it comes to spending $$$ for peace-of-mind. (Chalk it up to a lesson learned. BUT, the next time you buy a home, new or used, have it inspected.) What you would have done was told the inspector that you're concerned about sub-standard material, or short-cuts. Give an explanation of what you mean so you're BOTH on the same page as to what to look for. Usually, it's quality of material and quality of workmanship.
The GOOD news is that all is not lost. You can still hire a home inspector to look the house over. Discuss what you want from his report. Chances are that unless you can prove you were deceived, or promised 'something else', you wouldn't win a lawsuit to have a builder replace styrofoam with wood. 'As is' is not fool-proof but it's not that cut and dry either. You'll have to make a case.
Go to About Home Inspections, and click the 'For Home Buyers/Sellers' button. Let the page load. If it JUMPS you to another page (as it did to me), click the BACK button on your browser. From there, among others, is the answer to the question, How do I find a Home Inspector? Make sure the inspector is ASHI-approved. Again, be clear with what you want to get out of the report. Make sure you get your report form an inspector WHILE THEY'RE STILL ON-SITE. (Don't wait a week for results because you want the inspector to review the report WITH YOU, ON SITE, and ask any questions while he's there.) They do give estimates for repair/replacement of things. (IF he offers to do the work, it's conflice-of-interest and report him immediately to ASHI.) From the 'tone' of your Post, I say go for it!