Memorial Certificate
This is a certificate (memorial, honor of, appreciation, recognition, etc) on which a photo of the donor's pet (or pet of someone for whom the memorial is being donated) appears with a short narrative of appreciation indicating that something of value has been given to the organization.
It should be an attractive document, suitable for framing, and could actually be given framed if desired and the costs are reasonable.  It works because it appeals to a person's need to be viewed as a humanitarian, as a nature lover, and everything that symbolizes good about humanity. 

Done on a home computer, the cost is minimal.  Time is a little more pricey.  Scan a photo of the dog or cat to be memorialized, include the organization's logo for full effect, and be very sure to use high quality paper (preferably parchment).  The graphic design/layout should be professional looking.

One nice thing about this is that you can create a template for the document and only need to fill in the pet's photo (name & dates), and the pet's and family's name in the narrative section.  All other formatting, including the logo, remains the same.  Easy, huh?

Word of mouth is a good advertising tool here.  A newspaper ad, as well.  Also, it could be sold for a set amount, such as at a craft show, or with a discretionary donation by some other means.