An ancient search, finally concluded. I was in Costco the other day and found this salsa by Del Real Foods. Its short ingredients list (Roasted Tomatoes, Roasted Chilies, Salt) is a real strength, not just for what is is obviously lacking, but because it is good just how it is, and it won't add that cheap processed flavor to your food if you mix it in with your cooking. It is also good with chips. a little shredded cilantro, and a squeeze of lime.
Looking around on their site, I notice that most of the products from Del Real Foods have few to no onions. Del Real, I salute you, for attacking the enemy in their stronghold, and emerging victorious!
Update: Thanks to commenter Cinco, we now know of yet another commercially available place to buy salsa without onions. 505Chile.com sells quite a few salsa and chile products, several of which are onion free, these include:
Their medium salsa (yellow label but not green)
The red chile sauce/enchilada sauce.
The southwestern marinade.
Their roasted and diced medium green chile.
The chipotle and honey roasted green chile.