I haven’t need to purchase refills yet, but you can purchase from your box seller. I don’t see why you couldn’t use any vegetable grow supplement.
I have bought some of the sponges here on Amazon. Haven't found any basket replacements. I usually harvest one plant and reuse.
Sorry, I don't know the answer to that, however, what I DO know is that it's A LOT of work for very little results...I wouldn't waste your time and money on this product...also it stopped working after only one use :(...VERY disappointing!
I just added water & the solutions while growing my lettuce with very good results. I didn’t completely change the water, just added to it whenever it got low. That hadn’t happen often.