Custom Post Type Filter by Custom Field Values

I am building a review site and I have set up a custom post type with several custom fields that are filled out by users. I have been looking for a plugin or possible way to add a filter or sort to show the posts ordered by these custom field values, in ascending and descending order - like a product sort for a store only in this case the products are reviews. I haven't been able to find any plugin that will solve this problem, and any sources I have found online have yielded unsuccessful results. Any help is much appreciated!