How do I render this code in block/node page?
You cannot use that code in a node or in a block, since in both the cases the $page variable is not available. It seems that code is taken from an implementation of hook_page_alter() or adapted from code used by an implementation of hook_page_alter(). In fact:


hook_page_alter() receives $page as argument
The documentation for hook_page_alter() shows that, for a node page, $page['content']['system_main']['nodes'][$nid]['body'] is the content of a node body


If rc_core_item_avail_form is a node field, in a block you could use code similar to the following one.

// Load the currently shown node.
$node = menu_get_object();

// Render the field.
$elements = field_view_field('node', $node, 'rc_core_item_avail_form');
print render($elements);


In the case the field could not be visible to every user, but you want to make that field visible to every user when it is shown in the block, then I would use the following code.

// Load the currently shown node.
$node = menu_get_object();

// Render the field.
$elements = field_view_field('node', $node, 'rc_core_item_avail_form');
$elements['#access'] = TRUE;
print render($elements);