The plan of Hexham you have probably seen is “A plan of Hexham from actual survey” made by John Wood in 1826. Getting a good-sized high quality copy won’t be easy. Original copies of it are extremely scarce. There is a good specimen at Northumberland Archives, the public County Record Office, at Woodhorn Museum near Ashington. I don’t know if it is subject to any copyright restrictions, but you could request a very good quality copy from them at a reasonable price. I believe the Archives reference is NRO 00324/G/5. They might have other copies of it – they would advise you. Email archives@northumberland.gov.uk
The plan was also published in facsimile in 1991 by Frank Graham in a book called “Wood’s Town Atlas of Northumberland and Durham”, but copies of the book are hard to come by.
Hope this helps.