I wrote this about two years ago for a dominant woman friend of mine who wanted to get a corset for her husband, so I figured I would just do a copy & paste job on it - since none of the information has changed and there isn't too much information out here on teh interweb (that isn't a spelling error, it is how geeks write it) about men and starting corsetry.
First, if at all possible, go somewhere and try a few real corsets on. Everyone has a different physique, and corset styles and shapes vary a great deal. How a corset fits and feels one person might be totally different from how it fits and feels on another. Everyone has their preferences for styles, and designers, and fabrics etc. Corsets for an experienced wearer can be a VERY personal thing.
With that said, there are two great websites for reasonably affordable off the rack (non-custom) made corsets. They sell several brands, but the two I recommend for starting are either by Axfords & Vollers - who are two of the two biggest and best off the rack companies, IMO.
I've ordered from both stores, and had no problems with getting the corsets promptly, or returning them if necessary (which I did once, when the style just didn't fit me at all - for a store credit to be used later.) The two shops I recommend are: Corset Connection & Absolute Corsets. The both carry many of the same corsets, sometimes one is a little cheaper than the other. But I have had nothing but great experiences with both of them.
Now, about the fit. Men have a lower natural waist then women, which makes it all the harder to compress at the normal height of a woman's waist. Some corset companies manufacture specific mens corsets - personally I loathe these. They generally have a very limited shape to them, and while I don't wear a corset to fool people into thinking I'm a woman, I want some curves. So, with that said, I go with these two styles that seem to fit me pretty well in general. Since men don't have breasts - keep that in mind when ordering. Every off the rack over bust style I have seen won't fit well on a man (as an over bust style) there is just going to be a too much room left up top. Custom corsets, or specific styles - like the straight front Edwardian might, but for simplicities sake... if you buy an over bust then plan to wear it as an under bust. My two recommendations for a starter corset are the: 1891U style & the C130 style. I find that the bust area of the 130 is well used to give me added room for the rib cage - so I wear it as an under bust.
I have worn both of these styles in many several sizes. I wear them both as under bust styles, although the top of the C130 I wear right at my nipple line, while I wear the 1891U a little further down, just below my pectorals. By wearing the C130 (which is an over bust style) as an under bust, the room where the breasts normally fill allows me extra room for my rib cage and makes it quite comfortable.
Again, if at all possible, go to a store where you can try them on. Remember, off the rack styles fit everyone differently, and as you are a man trying out a corset made for a woman, the laces in the back might not line up perfectly when it fits "snug" - however, that's a small compromise to make for a good fit IMO. And if you plan to sit with any regularity while wearing it, be sure to do that when you are trying it on - because the longer styles can really push down painfully on the parts where a man is, that a woman isn't - get my drift? ;)
Just a little more info: Three other companies that make off the rack corsets that are commonly seen are, Versatile Fashions, Victoria's Secret & Fredricks of Hollywood. Both the VS & Fredricks styles are actually corselettes (very light corsets) and aren't good at all as real corsets. They are cheaper and probably just as good for a night or two out, but if you really want a corset, I wouldn't recommend them. The boning is plastic so it won't shape you significantly, and the stitching in the corset isn't designed to withstand the pressure that lacing up a corset significantly would put on it. The Versatile Fashion brand ones are more expensive, but IMO are not as well constructed when compared to Axfords & Vollers. Sometimes the Versatile are frayed at the seams in the store just from people trying them on. On the plus side, they use spectacular fabrics, so if money isn't an issue, it might be worth looking at their corsets. Although for that cost, I would look towards a custom corset myself.
Good luck shopping & have fun corsetting. And for more information or support (or just because you want some more wank materials you freak!!!), well then checkout the Male Corsetry Blog.
PS: The lucky guy in that first picture isn't me. I won't show my face because my privacy is too important for me. Either way, that photo is by "moonpanther". I tried to contact her (him?) but got no reply. If (s)he wants me to remove the picture, just contact me.