You could read my signature for a few. ;) I just did a search on eBay for "Samsung TiVo," and many like mine came up. Note that the SIR-S4040, 4080, and 4120 would all use the same InstantCake (and PTVNet) image; their only differences are in the included hard disk sizes. The Philips in my signature was from the [expired] "DVR4ME" promo, which I got for $16 shipping from DirecTV (the R15 was a free "bonus" from the installer who came out). Note that the DSR704 can be purchased from PTVUpgrade.
Don't get an R10 (it's not easily hackable), or any "R" after that (the R15 is not a TiVo).
Keep in mind that some of the units on eBay don't have access cards, and reps are usually not willing to let you use one from another unit. Even if a used one comes with an access card, it may have $100 of unpaid PPV movies on it. DirecTV will usually give you free programming and/or a discount on your bill if you have to swap access cards (they did both when I had to get an access card for one of my Samsungs).
HR10-250, Samsung SIR-S4120R, SIR-S4080R, 2 Philips DSR704s (one connected to a Slingbox A/V). One box used the "Zipper"; rest used PTVnet to enable networking features.