Replacing your AC Adapter or AC/DC Combo adapter
I have an R51, which came with an AC adapter, FRU P/N 08K8209. I supplemented that with an AC/DC adapter, FRU P/N 73P4503. so I could use the R51 in the car.
When the AC adapter started to die (cable getting flakey near the plug for the computer), I called Lenovo's 800 number to get another AC/DC adapter. I was told they are not readily available and quoted a high price.
A bit of googling and scrounging on Lenovo's web site revealed that Lenovo offers a newer "slim" AC/DC adapter, FRU P/N 41R0140. This comes for $119 plus local sales tax, much better than the $200 or so the support tech quoted for the AC/DC adapter. It comes with three different interchangeable tips, to provide 20V at 4.5 amps, 19V at 4.74amps, and 16V at 4.5 amps. The latter is appropriate for the R51.
There is a dual charging cable for the adapter, which lets you charge a second device, e.g. a mobile phone. Tips for a number of mobile phones are available.