I also find that parts availability depends on how close a big race either is or was. Right before and right after the worlds warmup for example, Carolinas RC was out of stock of most of the common buggy parts. This isn't an issue with them at all - just a function of the demand being higher for those parts at the time.
Like I said - plan ahead a little and parts availability shouldn't be an issue. It is good practice to keep a couple sets of the following in your pit box at all times:
- Front/Rear lower arms
- Hinge pins (upper and lower, inner and outer)
- Front/Rear lower pivot blocks/hinge blocks (inner & outer)
- Wing stays
- Front hub carrier pivot bushings/screws
- Rear hub carriers
- Suspension arm bushings
- Wheel nuts
That is probably the most common stuff to bend/break/lose. If you have a Losi add in a rear center shafts and a pair of CVDs. Most of that stuff is cheap.
If you have that stuff in your box at the track you shouldn't need to worry about parts availability. If something breaks, replace it out of your spares and either get new ones at your LHS if they stock it or order it online when you get home.