Here's what I've found - and I've only scalped ~10 times...
The closer you get to gametime, the more ticket prices drop. During the morning/day - most scalpers are on the strip or are near the UC side entrance facing Neyland.
Around the time the band heads to the stadium, scalpers are all around the stadium, from Gate 21 down to the river. If you have patience, these are your best deals - but not necessarily your best tickets.
Remember that there will be plenty of inventory even past kickoff - so don't stress that you won't get a ticket.