Former mortgage lender and underwriter speaking:
This could be REALLY hard! Though it's good that you have zero debt, BRAVO...sure wish I could say that! It's important to still have activity prior to applying for a loan. IE: Keep one credit card account open, if you don't want debt or even to use it, pay one consistent auto withdrawl bill with it every month and then just pay it off-create activity.
I would say your best bet is going to be a small town, local bank that you've done business with-if this is the case. They can create a history for you and possibly step outside the lines a little, particularly if they keep their funding in house. But, you need a history with them, accounts, loans, etc to create enough of a picture of collateral if you will. This isn't always the case, but I worked for a family owned bank for several years and for VERY loyal customers-we board approved this once or twice...still very rare, you need a relationship with the bank and they need control over their lending-which is hard to find.
Any larger bank or mortgage lender or 3rd party who purchases the loan and approves the loan based on their guidelines is going to want to see credit of some sort-you've got to have a score, period. Sad to say, that's the catch 22 of credit, you need to have a history of it, but you need to be responsible with it.
You might have to put it off for 6 months-a year and create a credit history again-which is going to mean probably 2-3 accounts being active, not carrying balances, but being used. If you had car loans, credit, etc in the past-they should be able to still see that and use it-unless it's been say over 5 years since you paid everything off and used any kind of credit or loan.
So sorry, I know-quite frankly, the rules suck. Talk to the loan officer too, there may be some new tricks, I've been out of the field for a couple years now.
Good luck to you!