Look at the commercial truck conversion companies in your area such as "Monroe", other re-seller's who are actual dealers for these caps or other commercial vehicle conversion companies who outfit a lot of trucks for industry. These companies contract with many different industrial companies such as ATT, Sprint, local utilities, government agencies and the like. They also decommission these same vehicles when replaced and as such may still have the USED and no longer needed truck accessory parts such as these fiberglass caps that have been removed from the older trucks.
You will be surprised how many of them are out there and available when you start looking in the right place.
You biggest difficulty will be your requirement of looking for a cap for a Nissan Frontier or Toyota Tacoma. Most of these types of fiberglass caps are manufactured for and installed on full size trucks. Not saying you won't find one however your mid-size truck requirement will exponentially increase the difficulty of your search.