you'll have to fetch them from tagdata yourself.
A combo of TMPL->assign_variables() and preg_match() on
the keys/tags you're looking for.

You can see an example in a small calendar class i made: https://github.com/GDmac/calendar_entries/blob/master/system/expressionengine/third_party/calendar_entries/pi.calendar_entries.php#L52-L68

you'll have to fetch / process them from tagdata yourself.
$this->EE->TMPL->var_pair will have an array of var-pairs you can walk.
Then compare / preg_match() them on the keys/tags you're looking for.

You can see by looking at an example in a small calendar class i made: 
Calender_entries (github, highlighted)