It really depends on what you want to accomplish and how many amps you need access to via the switcher.
Do you have separate cabs for each amp or are you trying to combine amps in to one cab??
For the former, I have an older Mesa High Gain Amp Switcher that works really well. 4in and 4out. Each is an amp/cab set-up.
Egnater used to make something similar to the Mesa as well.
Also, there is the Little Lehle for 2 amps and Lehle Dual that can actually do 3 separate amps. Also great.
I haven't owned, yet RJM makes some units capable of switching amps as well.
Then you can find a bunch that will A/B/Y two amps at a time.
Some other current options:
Palmer
Framptone
Radial
Voodoo Labs
Biggest things are isolation, phase, ground lift so there is not hum.
Best of luck.