BuildFastFormula Vasoblitz or Fullblitz are great pwos if you're looking for nitrates. The nitrates take about 7-10 days to build up in system. Then the workouts are totally awesome!
FWIW Nitrates should provide some benefits immediately, but other benefits do build with time. Vasoblitz is a pretty solid formula and I do think when used Nitrates should be used daily (most of the time) and not only pre-workout, unless you workout nearly every day (which BFF recommends daily usage of Vasoblitz so that is a plus).
I don't want to flood this thread too much, but Nitrates have tons of benefits, but there are some caveats people should keep in mind for maximizing effectiveness.
I know some people worry about anti-oxidant intake around workouts (with probably good reasoning), but co-ingesting Vitamin C with Nitrates is generally a good idea (to help with potential general health in mind).
Arginine and Vinpocetine help with Nitrate tolerance so they may be warranted inclusions. The amount of tolerance, etc can be debated a bit so not always a huge deal (especially if you cycle things every so often), but this is a slight plus in regards to using Arginine Nitrate versus other sources.
Nitrates take a little while to peak in the blood stream, so ingesting it a little further from your workout than right before might make sense, or just dosing daily and then you will notice stronger effects down the road as rodefeeh mentions here is worth keeping in mind.
Finally, conversion of nitrate happens somewhat in the mouth/saliva so trying not to dose too close to brushing teeth or mouthwashing is worth keeping in mind. There was an interesting thread in the past where I believe Poison brought this to light to me, which is a pretty cool/interesting interaction we don't normally see!