This article on WP Beginner covering how to add custom post type selections into an advanced search form.

Fearless Flyer writes an article covering searching a custom post type with specific taxonomy terms applied to it via dropdown menu selects.

A full on thread on WordPress Stack Exchange covering advanced search across multiple post types and taxonomies.

Creating a Taxonomy Picker Plugin for WordPress taken from Squidoo.

Kevin Chard on WP SNIPP covers how to list WordPress posts in a dropdown select menu.

Spruce Interactive write an amazing article on creating a double dropdown select menu filter for taxonomies applied to a custom post type.