Datacolour do a range under the trade name of
Spyder
If you are running Windows 7 then you need the Spyder 3 but if you are running XP or Vista then a Spyder 2 will do and these come up from regularly on ebay. There are different models within these ranges with the more sophisticated ones doing printer profiling as well. I notice they now have a Spyder 4 but the first I heard of this was today when visiting their website before writing this.
My father in law is an ex pro photographer, does our printing for us and has a Spyder 3 which we all use as there is no restriction on the number of computers you can install it on. I have three computers in the business and they are all regularly recalibrated. As I am still running XP I will pick one up on ebay as soon as I can get one at the right price.
A new Spyder 3 can be bought online for £65 for the basic monitor calibration only version (Spyder Express).
If you are using an online printing service then I would not expect too much colour accuracy especially if you are using one of the budget price outfits. If you have any photo studios as customers I would ask who they use as they tend to stick with the labs that produce the most consistent results. Two of my pro photographer customers use One Vision -
http://www.onevisionimaging.com/
Another factor to consider is your customer will have probably given you the image they have looked at on their own monitor which was most likely not calibrated either.
Colour management is a whole new ball game!