If you check at your local AutoZone/PepBoys/Advance parts places or better yet, NAPA, you should find a variety of universal brackets. Be sure to take sketch with measurements of your battery and any possible mounts points so you can match it up with what they've got on hand.

Another good place for battery goodies is a commercial truck supply or truck stop.

Do take a good look at your electrical though, as a drain enough to kill your current battery after three days will eventually ruin your Optima as well. No battery is a cure for an electrical problem! If you have a multimeter, take a reading between the positive battery cable and your battery's positive post and take note of the amperage draw. You may find you've got a load somewhere that you shouldn't, and you'll need to track that down and get it on a switched or isolated circuit.