A character may commission a suit of plate mail specific to his or her body mass and proportion. The in-game AC benefit is the same, but the character is able to move as if it were chainmail: the weight is evenly distributed and more expensive material may be used in the joints, etc. The aesthetics are altered as well — plate mail corresponds historically with plated (splint) mail or laminar (banded) mail, while field plate corresponds to a range of stronger historic armors.
The total cost of field plate is 2000gp. All field plate is crafted to the specifications of the wearer; it is not available "off the rack."
If bought outright, it will take one month to craft — possibly one week less if it's cheaper, or one week more if it's more expensive, depending on if a carousing save is used. If carousing rules are used, the character will only have to make one carousing save, but multiple carousing attempts will be needed to complete the armor, as it won't be finished until the 2000gp figure is hit (or more). There can be breaks in the middle to carouse for something else, but this will obviously result in finishing the armor more slowly.
Note on demi-human armor:
It may turn out to be less costly than normal due to a smaller suit being theoretically quicker to craft, thus having less of an impact on the craftsman's profit margin. It may also be more costly because the proportions are unfamiliar and thus tricky, requiring more time than usual to get it right; or because armorers are less inclined to cut a generous deal with a foreigner or a non-human. We can determine this either with a random roll if you want to buy it outright, or with a carousing saving throw.