There is an entire thread (and its long) on multi-engine training on here.
I'm not sure what you're looking for (i.e. nicer planes, a certain location, cheapest cost, etc) but if you're basing your decision mostly or fully on a cost basis, that thread goes into detail about the cheapest multi engine rental rates in the country and names several that top the list. I believe they narrowed it down to Vetter Aviation in Montana. They have unbelievably cheap multi rental rates. The plane isn't a cream puff, but hey, it's multi time, and it's cheap. View the thread to see for yourself what people had to say. Someone went out there and then talked about it on here. From what I remember, they had pretty good things to say.
There's one catch- it's literally in the middle of nowhere, so good luck getting there (and getting out for that matter).