Ordnance Survey Index
The early editions of these OS maps are available at The Lancashire Record Office.
They have 6-inch sheets from the 1840s and 25-inch sheets from the 1890s.
By consulting the index map below you can find out which map(s) you need.
OS Index for Lancashire - 6-inch and 25-inch Early Edition Maps
The index map, as well as providing the sheet numbers for the above maps, is also an excellent Parish map of old Lancashire.