I had no problem ordering directly via email from Stouffer.net . They don't run a web shop as such, but still good service and quick delivery, given the fact that they sent the step wedges to Sweden. The shipping and handling from the US to Europe was $10.
For my work the 4X5 the most useful one and is called "TP 4 x 5". They are sold in 21 and 31 step versions (where the 21 steps have .15 or 1/2 step intevals and the 31 step has .10 or 1/3 step intervals).
If you are using 35mm or MF film the step wedges are made in those sizes too if you plan to use them from the enlarging head, depending on your enlarger size. (The advantage here is that you "calibrate" with your personal enlarger setup.) The 4X5 size is the most useful if you are going to contact print the step wedge directly on the paper. In short, bigger is better, but it all depends on your needs and your equipment.
Finally, if you're really "fuzzy" about exact readouts, they sell calibrated step wedges too. I.e. every step is measured with a correctly calibrated densitometer and the read outs are recorded in a sheet which comes with the step wedge. Now, most users find the normal "non-calibrated" step wedges good enough. (The calibrated step wedges cost about $9 more.)
//Björn