For customers in the US, there are 3 major distributors. Who you choose depends on what you want since they handle different things.
www.towerhobbies.com
www.hobbypeople.net
www.horizonhobby.com
All 3 have an excellent reputation for reliability in the US. I have no idea how good they are if you are not a US customer.
There are dozens of others. Some have better pricing than the big 3 on some items. Some sell things the big 3 do not carry. Some break up sets and sell the components in any combination you want.