I always have a core I order from.
For Kits & Parts I always try to order from Steve at Speedtech RC.
For everything else I use:
Stormer Hobbies (Great Everything)
A-Main Hobbies (Great Off-Road Selection)
Tower Hobbies (for big $$$ part/supply orders)
Ultimate Hobbies
Nexus
Last Case Scenario:
Horizon Hobbies (too expensive)
Overseas like Japan or HK are great for parts.
Do not order anything big as shipping is too expensive.
Rule of thumb is if it is bigger than a shoe box or really heavy, do not order overseas.