Dawn, you haven't mentioned your dog's age. The older he is, the harder it will be to get him to accept something new as a replacement to his obsession over this one toy.
I did a search on eBay which would be the most likely place to find something discontinued. Unfortunately, it looks like the sale is about to end, so I hope you get this post asap
Lot of (2) Hartz Mad Maddogs Latex Deluxe Hamburger - eBay (item 220190843265 end time Jan-13-08 18:26:16 PST)
Here are some links for latex hamburger toys for dogs. Although these aren't made by Hartz, they may be close enough to the look of the original that he just might accept it.
Tiny Tails Latex Hamburger - Vo-Toys Inc
LATEX HAMBURGER - 75726005356
Vinyl Hamburger- Dog Toy 4 in. Supplies for Dogs
Product Catalog > Pet > Dog Toys > Chews - Latex Hamburger Dog Toy - Agway
Scroll down to the hamburger on this one:
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Some dogs do show an obsession with just one toy and no matter what we do they refuse to accept anything else. Some dogs aren't interested in toys at all. It is just one of those things that we have to accept if we have tried everything we can think of to get them interested. Have you tempted him with a chew toy and then taken it away after just a few moments, putting it out of reach but in a place where he can see it? Sometimes, continuing to limit their interaction with a toy and reintroducing it again later, repeating this procedures a couple of times a day, over a few weeks period, can create a "must have" kind of effect in a dog. If you haven't done this, try it and see if it works. Another idea is: have you tried a kong toy made for small breed dogs? They have a little hole in them in which you can place your dog's favorite treat. If your dog likes peanut butter, you can rub a small amount inside of the kong. The favorite treat will encourage your dog to interact with the kong immediately and possibly allow you some relief in having to find that ONE toy that he responds to.