How can i get dimensions for different mechanical components?
I want 2d drawings for creating 3d models.Without those dimensions, i cannot model parts or components.
Sure you can!
2d drawings are certainly not needed for modeling parts, or components, in fact most of the time the 3d part is modeled first, and the drawing is created from that.
If model creation practice is what you desire, just start modeling, and choose your own dimensions for features. You don't even really need dimensions, just sketch and model 'unconstrained' (trust me, it's a very liberating feeling)
If you're looking for simple dimensions on stock parts, like bearing id/od/thickness, or screw diameters and lengths, things of that nature, most of the time you can get those dimensions out of the suppliers catalog.
Usually those guys also have 3d models available for download, so re-modeling hardware and such is a waste of time these days, but if you're just looking for practice you can try part supplier catalogs.
Also, try googling '2d drawing' and clicking the images tab, it will give you hundreds of drawings to work from.