Does every drink bottle fit into every holder? I have a Specalized drink bottle holder and was always using normal 750ml plastic bottles, but they never fit very well. I was looking online for water bottles and now I'm wondering if all bottles that can be found online (e.g. on bikester.com) will fit into my holder or do I have to be careful with what I buy? Are there different sizes of bottles and holders or all they standardized?

Thank you in advance.
The inexpensive bottle cages can be bent to make them a little smaller for a tighter fit. If the type you have is a closed  loop type you can add rubber bands to the bottle for a more secure fit if it is loose. There seems to be a lot more size variation with generic "sport bottles" than the bottles marketed to cyclists.