Best "store bought" trace element mix?

2 part answer for you.

First: You need the rest of the nutrients plants need to grow, not just micro nutrients. These are Carbon,(which you have in the form of Excel) Nitrogen, Phosphorous, and Potassium. Your plants also need Magnesium and Calcium, but this is probably provided by your substrate. There are many sources for cheap aquarium fertilizers, a few to start with: Green Leaf Aquariums, www.aquariumfertilizer.com. My advice, call Orlando from Green Leaf, he is a great resource, and a friendly guy. Tell him what you want to do, and he'll get you what you need. This route is far cheaper than buying prepared fertilizer mixes.(Seachem, ADA) There are many great plans for dosing that are very easy to follow - take a look at the fertilizing forum for ideas.

Second - Trace mixes are pretty subjective, as far as I can tell. I've used CSM+B ( a cheap, commercial dry mix), Seachem Flourish, and Tropica Plant Nutrition. I really like the Tropica micros, and have seen great results using them. But I've also seen tanks that make me weep with envy that just use CSM+B. My advice, start with CSM+B, then if you're not satisfied, try something else. But without the other macro nutrients, it won't matter which micro you use, you will still have problems.

Good luck!

what do you mean by water problems? I saw in your first post you mentioned something missing. Tap water doesn't have what plants need to live. And by providing only traces (which iron is also), they will continue to do poorly. An example that might help:
Imagine getting a puppy, and only giving it water and vitamins (trace elements). With no food, it will not grow, and will die.
Plants are the same. Without food, no matter how much vitamins or water you give them, they will eventually starve. Here is a link to a good starter package, with enough fertilizer to keep you going for a long time. Here is a great plan for dosing them. Since no CO2, cut your dosage in half, and only dose 1 or 2x/week. Keep the water changes the same. Also, watch your light. If you have too much, without the carbon plants need to photosynthesize, you will have a prime recipe for an algae outbreak. And excel, while able to provide some carbon, cannot keep up with the demands high light will produce.
If you have questions, let me know.