Brad, I've not seen it anywhere like Home Depot - also, unless you're planning on not having your construction last more than about 10 years, you should look at EPDM rubber - more expensive than neoprene, but lasts closer to 25 years under the same circumstances. 60 duro is typical, 50 will work also.
One of the reasons the Auralex U-boats seem expensive is that they ARE made from EPDM for longevity.
Here's a possible source of EPDM, but you'd have to contact them to see if they sell small enough quantities -
well, ive got two kids in highschool right now, and will be moving to a smaller place once i have to sell my house to pay for college!!!! so quality is not a problem for me.