The Canon 100 L Macro runs about $1000, so that's out (although it's very good).
A lot of people rate the Sigma 105mm macro just as highly, but it's just as expensive.
The Canon 100 USM (non-L) Macro is about $600, so that's probably out too.
There is an older Canon 100 non-USM Macro, which i've seen go on fleabay for $300-400. I'm not sure how well it performs, afaik it's not as good as the USM or L.

Sticking with Canon, there's also the EF-s 60mm Macro, which by most accounts that i've read is very sharp. Of course you don't get IS, but it depends how close you want to get and how much light is around.

Anyone of those 4 options are good, and are all Dedicated Macro lenses, that will focus anywhere from 1:1 to infinity. There's also the 50mm Macro, that will only go to 1:2, and the 180mm L macro (which is just huge, I wouldn't use it for macro without a tripod, but if you do it's great). The good thing about them is that they also make great portrait lenses.

Another (slightly more annoying but cheaper) option is to get any prime lens, and a set of Kenko Extension Tubes (B+H has them for $200, I paid closer to $90 in Hong Kong, you can probably find a good deal via fleabay). Then you can choose from the 40mm Pancake ($200), 50/1.8 ($100), 85/1.8 or 100/2.0 ($3-400), or even use your 55-250 (zoom lenses with tubes is a bit weird, but you get used to it). But the annoyance is that to get down to macro distances, you have to pull the lens off, put the tubes on, and put it back on, then you can't focus to infinity without taking the tubes off again. For a 'shoot' where you're always doing the same thing, it's fine, but for walkaround it gets annoying...