Mad Men suits are one of the hottest styles around. The show has definitely brought a lot of popularity back to 60s fashion!
The look of Don Draper and Roger Sterling has proven so popular that Brooks Brothers actually designed a line of suits based on the show's designs. They actually worked directly with the show's designer Janie Bryant to capture the same style. Its got the slimmer look and tight lines. Unfortuneatly, its no longer available but you might be able to pick up a used version somewhere. They sold out extremely fast even at $1000 per as only 250 were made. I think they were nice looking but the hacking pockets rubbed some people the wrong way.
Banana Republic is the last to add a Mad Men suits promotion. They have a new promotion for the show with suits and attire. I haven't seen them in person so the jury is still out.
If you missed out on the Brooks Brothers promotion, some of the current models from Hickey Freeman and Ted Baker are similar. Look for a nice worsted wool two button from HF or a Ted Baker 3 button. Stick with mostly greys, but an occassional blue can look OK.
Besides the slimmer look, the period also saw skinnier ties and lapels, along with plenty of smoking! Most of these suits also had higher armholes as well. The shirts were crisp, often Dior Homme.
Most people think of the Don Draper look from the show but Roger Sterling was actually more of a 50s throwback fashion guy. Things like the fedora were a giveaway.
Besides a nice fedora, there's some other things you can include to complete the look. A nice overcoat with a hat looks great. You can also go with a trench coat look. Also, consider adding a tie bar as another nice touch. A pocket square is a must.