By the normal way, you can get a block by using object Layout.
For example, in a controller:
$this->getLayout()->createBlock('module_name/block_name');
It is the same with a block or a template file.
But if you want to get a block in a helper, model or anywhere, what way?
In this case, you can use:
Mage::app()->getBlockSingleton('module_name/block_name');
Related Tutorials: