I'm not certain by any means but I'd bet a beer that ANY usb bluetooth dongle works. Since they can be had on amazon and ebay and hte like for $1.99 - I'd try one of those first. Or if you are less sure then buy locally (staples, bestbuy, maybe the rat shack...) where you can return if it doesn't work. If I can recall the instructions with the remote (likely available online) explain how to re-pair the remote to the dongle.
A generic usb bluetooth dongle might need you to hooked to your pc/laptop to run it's pairing routine- so might need to pair to the new dongle when hooked to your pc and then remove the dongle and place it in the tivo.
edit: here's the pairing instructions - would need to modify to include generic usb dongle:
notice these relevant bits
The TiVo Slide remote ...may also be discoverable by other Bluetooth devices; however the remote ...(is) ONLY supported for use with ... each other
so they acknowledge that they probably pair with other devices but wont support it.
Also- since the instructions dont mention needing to enter a PIN- I believe that means it uses "Secure Simple Pairing (SSP)- Just Works" which is in only the later versions of bluetooth (2.1+?)- so make sure the dongle you get is a newer version. Might be best to find a 3.0 to be sure you get the latest features. (I think a dongle that supports this probably just would need to be plugged into the tivo and press it's pairing button while you enter the pairing mode on the remote per the instructions above from tivo's website)