Filter option for Gallery listing

Hi,

I've got a WP theme that supports post listing by category in a gallery format, eg 5 rows x 3 columns. Question is how can I add addition filter options for the user to further narrow down the listing by custom fields or tags that I specify in the respective posts within the same category.

I am looking to create something like what you can see at the url below:
http://asdsingapore.com/wp/adopt-a-dog/

The developer of the theme is charging me US$100 for the customization. As the website I am working on is a non-profit organization, I hope to get from free advice here on how to proceed forward.

Many thanks in advance!