Search-Into-Subcategories plugin allows you to make a select-search shortcode for your own categories. You can use if everywhere you want and make a tree structure for your categories.
[search-into-subcategories]

Arguments you can pass parent_category Default is 0 You can list only sub categories of the category id you give here. With 0 lists alla categories max_depth Default is 2 How many subcategories to display, at least 1. search_input Default is 1 Display a text input for search. labels Default is Labels for the inputs. Give them in the position you want with ‘|’ as seperator. example: Category1|Category2|Text search_text Default is Search The text to display in search button hide_empty Default is 1 hide categories without content, set to 0 to display all! exclude Default is Category Ids to exlude from listing. Separate them with ‘,’

Example: 6,7,13

Example: [search-into-subcategories parent_category=0 max_depth=3 search_input=1 labels=cat1|cat2|cat3|Text search_text=Find hide_empty=0 exclude=7,6 show_date_ranges=1]

Demo:

Link: http://wordpress.org/plugins/search-into-subcategories/