Everyone is going to tell you the same thing, go to a shop, try on lots of different boots with the boot fitters help. Once you know your size you can then maybe find an older pair online, but you need to make sure nothing has changed between the new and old model, such as liner/shell.
But definatly, go to a boot fitter, one helped me realize i needed an arch supporting footbed, once I put a footbed into some boots I was trying on, they fit like a glove, I wouldn't have gotten such a good fitting boot without knowing I needed one