The OEM 'dealer add-on' was the VIP RS-3000 alarm. Later, they went to the RS-3200 but I don't know how that goes from model year to model year.
Anyway, the RS-3000 alarm offered key fobs badged with the 'Toyota' logo on them and 'Toyota Security' LED dash plate.
I'm sure there are a few of these floating around in junkyards but I'd rather get a new system, since the price of a new Viper alarm isn't that bad. I've used Viper before on older cars, though, and they were nice and easy to install.
I THINK you could get the body control / theft module from a similar car with keyless and make it work, but you'd need the dealer to code the new module to work with your car. I found one for my Sienna for $50 but then I'd need fobs and then the dealer would have to agree to activate it. $$$$$
Let us know which setup you go with, chasman. Interesting topic.