If want to buy a Tivo at the lowest price and you have lots of time and like thrift stores, the cheapest place to buy a Tivo is there. Or at garage sales. Make sure you take a Tivo remote control, power cable, Tivo 800 number, cell phone, TV cables with you. You will probably need them, as I would say that less than 10% of the Tivos that I have found have that equipment. I am so cheap that I usually don't call Tivo to check to see if they have lifetime, but that can be dangerous. As twice now I bought Series 1 Tivos that said lifetime on the systems info screen, but when I hooked it up to the phone line is changed to unsubbed because the lifetime had been transferred. . I can still use them as a glorified VCR, but I have 15 lifetime machines, so no need. So it is best to call Tivo to see if the lifetime is still good.
But if you don't want to waste a bunch of gas driving around town, then try ebay, but use your brain.
Many people buy Tivos at half the going ebay price because they use the 'buy it now' feature on some Tivos listed. Many times people will list a Dual Tuner Series 2 Tivo for $100 instead of the going price of $200. And many times the lister will not mention that it has 'lifetime' in the title. Those Tivos usually go for about half the price as Tivos that say "lifetime' in the title. So use the "Include description" box just below the 'search' button, but put in "lifetime -fiber -netgear -replaytv -replay -Monitor -Video -Adapter" in the search field and only search in the Digital video recorder section. That will narrow down what will be listed. Otherwise a lot of garbage comes up. You can put 'Tivo' in the listing too, but some people don't put 'Tivo' in their listing so you will miss their lifetime Tivo if you put 'Tivo' in.
There are other things that you can do on Ebay to find that cheap Tivo with lifetime. Some of them are very tedious and most listers wn't answer questions adequately.