Where can I find more Desert Spores?

You should be able to find 14 of them (source), it's for the Xbox 360 version, but that shouldn't make any difference.

• 2x from getting the recipe
• 2x by Joe's swamp
• 1x jump to mailbox (Mailbox challenge)
• 1x to the left of Bash TV
• 1x by Dr. Kvesir's mailbox
• 1x by the refuse "treasure" pile of the Outrigger Sewers (entrance located to the left of the parking poles leading to the Radio Tower)
• 1x by the Wellspring sewer entrance
• 1x by the northern tower that you have to resupply
• 1x by the Abandoned Distillery entrance (left side in the sunlight while heading towards the entrance)
• 1x to the left of Power Plant
• 1x at the sixth mailbox from the 3rd challenge
• 1x under the bridge leading to Subway Town, just past the sandbags that stop you from driving down there.