Some of the information on the HUD-1 form would have been useful to complete the income tax paperwork the next spring. It would have had numbers for Taxes, and interest that were addressed at the settlement. It is possible it is mixed in with the next years tax information.
If I needed a HUD-1 form from 15 years ago, I wouldn't ask the real estate agent, I would ask the settlement company. They might have a copy of the paperwork. They might have to retrieve it from an archive, so it could take time, and they could charge a fee.
The local government probably doesn't have a copy of the HUD-1, but they do have paperwork documenting the sale price when the transaction took place. I know that the jurisdictions in my area have on-line the tax appraisal information going back a number of years. They also list all the purchases because of the change in ownership, and many also list any name changes. You probably don't want a screen capture of the transactions page, but the tax office might have what you need. This is the same information that the title search company was retrieving for their report.
Question. Is there going to be capital gains? For a single person there is no gains unless the increase in price is $250,000. For a couple it is $500,000. I am ignoring any time requirements because you mentioned the purchase was 15 years ago. I am also assuming that it was never a rental property, because that would require a lot more paperwork.