Going back to the sourcing question: Find your local hobby store specializing in R/C vehicles. (This may be rather an interesting venture in the LA area…) Or the various FRC suppliers may have suitable ones. Two key things to note: You need to be fairly specific about what type of gears they have internally (metal is generally preferred over plastic for FRC use), and you need to know whether you’re getting a continuous-rotation servo or not (R/C tends towards NOT but that isn’t always true).
And for attaching stuff: Most servos, when purchased, will have servo horns or servo arms, a small screw, and a pinion (splined not toothed). Slide the servo horn/arm/disc over the pinion, screw the screw down the center until snug.
However… For FRC usage, servos are exceptionally low-torque. Use mainly as latches or for very low-power items. Or gang a bunch of servos together. I would say that use of servos is #4 on the list of least-used stuff in FRC (Nidec, Throttle, and Solenoid Actuator being the top three).
We source our FTC servos and hardware from servocity. They have a great selection of servos along with the tech specs to pick the ones you want (i.e. rotation angle, speed, torque. They also sell servo horns that integrates well with the actobotics hubs which match the bolt pattern for various sprockets, pulleys, wheels or other devices that you may want to use to integrate the servo into a mechanism.
I highly recommend these servo blocks to stabilize the servo output horn so that you are not side loading the servo itself.
We have not used any servos for FRC, but we have discussed using them at various times in the past few years. I think they could be useful for some applications, but they certainly seem to be more appropriate for FTC sized robot applications than FRC sized applications.
There is also the 393 motor from VEX. In FTC, this counts as a servo. I’m not sure what the ruling is for this motor in FRC. It is significantly faster and more powerful than the other continuous rotation servos that are available. It does have a unique power supply and motor controller that are needed to make it work. Also, the output is the VEX square shaft which does not integrate very well with commonly available parts (you pretty much need to buy the VEX EDR gears, sprockets, etc.).
Also Google “hitec servo” for a bunch of different styles. Always try to use one with metal gears. If you are using a standard size servo then powering it from the Roborio is ok. If using one of these brutes, then a REV power module is recommended.
As of 2018 rules, they need to be under $75.00 so be prepared to prove that.
FYI, I believe the Roborio separates the important power rails from the servo power rail. If you overload the servo power, it will report a brownout but won’t reset the Roborio. (needs verification).
Keep in mind that even brute servos are delicate. They can’t tolerate impacts, shouldn’t be stalled for any length of time.