I think it depends on how many diapers you plan to make, and whether or not a snap press would be useful to you for other sewing projects. I think having someone else add snaps would be great if you're only doing a few, but if you have a lot, the cost adds up quickly. From what I've seen, there's usually a charge for each garment, plus a charge per snap, plus shipping both ways. If you know how many you want to make, you can determine how much it would cost to have someone else add the snaps versus buying your own snap press.
Here is a list of other people that offer snap service.
Here is a snap press FAQ with pictures to show how to use a snap press.
I bought my snap press from
The Snap Store. Right now, they're out of stock, so she's running a pre-order special. You can get a snap press and a 3-piece die set (that's everything you need except the snaps) for $74 + shipping, and they'll start shipping out in about 2.5 weeks.