One of our senior Peak Testosterone Forum members (Prof) did some research and found a great study that looked at many foods, drinks and supplements in order to evaluate nitrate and nitrite content.  (It is available as a full study if you wish to read the details.) The best nitrate content was found in the following:
"Note that a desiccated vegetable dietary supplement (Natureï¿½s Way Garden Veggies; Natureï¿½s Way Products Inc, Springville, UT) had the highest nitrate and nitrite concentrations of any food tested, ie, 27,890 and 10.5 mg/100 g FW, respectively. Each capsule of this supplement contains 900 mg desiccated vegetable product, and label recommendations suggest a daily intake of 2 capsules daily, which equates to >500 mg nitrate and 0.2 mg nitrite per day."
Now the above sounds very good, especially considering that, as supplements go, this is fairly cheap. The entire bottle costs $8.95 as of this writing (5/7/2016) and so you the cost per 500 mg of nitrate is about $0.30. Obviously, this is considerably cheaper than buying bags of frozen organic spinach like I do at Trader Joe's or somewhere similar. .
However, here is the problem: I tried this supplement and it did not move the Berkely Nitric Oxide Test Strips at all! In fact, I took 5 of the capsules over a four hour period and measure twice at the appropriate time and the strips showed "DEPLETED," which is the lowest level of salivary nitrites. One of our senior posters observed the same thing and wrote: "I tested the Nature's Way Veggie Blend capsules this morning. I took 4 of them mixed in with my smoothie (my normal smoothie does not move the Berkeley Test Strips at all from depleted). My reading before the smoothie plus capsules was depleted. My readings 1 and 2 hours later were depleted with absolutely no change whatsoever in the color." 
NOTE: So you can try this product, but two of us now have had no luck with it. Here is the product here: Nature's Way Garden Veggies, 60 Vcaps. Also, if you don't know what I am talking about, then read my page on Spinach and Erections to get yourself motivated. These increase my nitric oxide so much that it dropped my blood pressure 10+ points. Note that this is why spinach (and arugula) are such powerful stimulators of arterial nitric oxide. For details as to how this works, read my Interview with Dr. Nathan Bryan and keep in mind that you can measure your nitrate levels using the Berkeley NO Test Strips. You can also read
Of course, one can't dispute convenience of just popping a couple of capsules. It obviously takes considerably greater effort to prepare and chew a bunch of spinach or arugula. That said, let's compare the above to the amount of nitrate in frozen spinach. I buy my spinach at Trader Joe's typically in 454 gram bags (16 oz.). A reasonable serving in my opinion is a fourth of the bag. The same study shows that spinach has 740 mg of nitrate per 100 grams of spinach. Thus eating a fourth of the bag will give you 840 mg of nitrates! This is about 68% more nitrates than the 2 capsules above. And arugula can deliver even more nitrates!
So why did I post about this? My point was this: stick to spinach and arugula! Both of these will deliver a good dose of nitrates and, furthermore, they are very likely safe as well. I always recommend consuming nitrates with low fat in your stomach and Vitamin C for reasons I discuss here: see my page Are There Any Dangers to Consuming Nitrates in Foods? for details.
1) Am J Clin Nutr, 2009 Jul, 90(1):1-10, "Food sources of nitrates and nitrites: the physiologic context for potential health benefits"