A lot of people get replacement from Amato Industries. Their website is rather confusing, so you need to be sure of what you want, before you start. If you just need a single item, you can order from Amazon (and you may actually be buying from Amato) but you have to pay shipping separately for each item, buying that way. My test kit info page has a link to a 1 pint bottle of R-0013 CYA reagent, one of the most commonly replaced items. You can buy any of them in large sizes, pop the tip off your dropper bottle, refill, and then pop the tip back in. Use clean hands, if you do this!

And do NOT try to get the large size of DPD powder; the Taylor DPD bottle has a desiccant coating that protects the DPD powder, but has limited life.