Hi Harley,
So I'm gathering that you have the HUD Certification label numbers, but no serial number?
If that's the case, ITBS is a good option, but I've also been able to ferret out the serial numbers in other ways at times. Just depends on how much information the seller can dig up. I've been able to pull the numbers from NM titles, insurance declarations pages, sometimes the county assessor information. I've even pulled old Real Estate Contracts and been able to find the serial number noted on them if the lawyer drawing up the contract was detailed enough. It's an outside chance, but it's worked from time to time for me.
ITBS is quick, and the database is great. It's just a matter of spending the time or spending the money at this point, I suppose.