Go to your local drug or grocery store. Look in the cleaning supplies section. Generally, the cheaper the product, the less likely it is to have surfactants, perfumes, etc., which you want to avoid. I went to my local London Drugs (which I don't think you have in the US), and just grabbed their brand of 100% ammonia from the household cleaners. All different brands will have different concentrations, so make sure to go slowly when adding it and make a few measurements with your test kit. I think I got a liter for about $2, so its cheap stuff and you'll never use that much.
If you're just going to be adding a betta you can probably just add enough to get you to 2-3 ppm of ammonia and then see do your test 24 hours later and see how much the plants have used up and add more accordingly. The biological cycle will still form, it'll just be slower and fewer bacteria will form as the bacteria won't me solely responsible for taking up the toxins. If you're planning to add more fish and depending on the size of the tank you may want to make more ammonia available in the tank (4-5ppm) so the tank is ready for a larger bio-load on the intro of fish. Or just take it slow and only add a few fish at a time after cycling.