...but we can not locate the original. I do have a copy with my original I-485 application. Will that be sufficient or should we request originals?
It's not govt. job and responsiblity nor they can give applicants a free pass on certain important documents if applicants cannot locate/find/submit those documents. The burden to prove and submit documents are always carried upon the applicants, and not upon the govt. That being said, you have to locate or submit the birth certificate of your wife, one way or another.
Now when you say that you have a copy of it in your original I-485 application then make the copy of it from your record of I-485 and send it to the USCIS. I'm sure you must have the full record to your I-485 application. And I'm also certain that USCIS must be asking a copy of it and not the original one. But it is advisable to obtain original one as well regardless USCIS is asking it or not, which shouldn't be the problem. Just make the copy of it from your I-485 application and give it to your wife so that she could contact the Registar office to get a duplicate one. That's it.
It's not clear who is the US citizen here. I mean, you or your wife? But since you have mentioned about USCIS asking your wife's birth certificate then I assume that she is a US citizen. In a naturalization application which is filed based upon 3 yrs rule of being married to a US citizen, a proof of US citizenship of US citizen-spouse is mandatory or a must. It could be a birth-certificate, naturalization certificate, US passport or etc. So, it's a part of the processing requirement. Sometimes USCIS asks the birth certificate of US citizen whenever they have a doubt in this regard.
However, it's always advsiable for applicants to take their birth-certificate with them at the interview even if their naturalization application is not based upon being married to a US citizen. Birth certificate of applicants is being asked just to make sure the correct name and date of birth of the applicants as many applicants have so much inconsistence and fuzz about their name thing. Thus, there are some District Offices wherein a birth certificate of the applicant is ALWAYS asked, similarly like in NYC district office wherein tax returns are ALWAYS asked regardless.
As for those immigrants who cannot obtain their birth certificate anymore then they would need to submit affidavits from at least two people who had the knowledge of their birth. Those people could be anyone, like parents, family members and etc. In addition, immigrants must need to submit a letter from Registar/authority which is called- 'non availability of birth certificate', which is easy to get. Most of time the reason that authorities provide on this kind of letter is that either their record has been destroyed, or they start keeping the records only after so and so date. Also, it would be best to submit any Batpism and school documents as well along with all these documents.
So, it's just about being proactive and fully prepared.