You can generally pick up a used one at e-bay but whether Nvidia will keep the GTX 460 on the market is doubtful, at best. Recently they pahsed out the GTX 470 and flushed inventory (I picked one up to sli it), and GTX 480s have only been rarely seen. Now Nvidia is getting rid of inventory of GF 110s the GPU in the GTX 580 & 570, by bringing out the GTX 569 ti-448. They are preparing for the launch of their next series, (presumably) the GTX 600 in Q1 or 2012.
You can SLI two different GPUs from the same GF series, e.g. the GTX 470 and the GTX 465 Which are both GF 100s) , but the performance drops to the lesser of the two GPUs. I have not seen where a GTX 460 was sli'd with a GTX 465, as the GTX 460 is a GF 104.