Im going to assume you have unlocked Krokotopia, so, considering Stone Block is rare in the bazaar, the best way is just to collect them lying around in Krok.
Another Piece of advice is to just pick up any reagent you see lying around, as you never know when you may need it to craft something!
Gilroy DarkTalon lvl 31
This is a great piece of advice! You might wonder, "when will I ever need 999 chunks of ore?" for example - consider this - you might not, but you might need a bunch of diamonds someday and with a transmute spell you can make diamonds from ore. Reagents take up NO space in your inventory so pick up everything you see. I would also suggest that you purchase every transmute recipe you can find which virtually all of the crafting vendors offer for sale. When you get to some of the higher level crafting quests you'll be glad you did.
There are some guides on reagents, but for my personal experiences, I would say look around various areas where they spawn frequently and in large amounts. The Shadow Palace is a prime place for farming many different reagents!
I suggest Krokatopia, you can really find them anywhere in the halls but I suggest the end of the halls where the boss sigals are. Also, sometimes (rarely) you can find them in the bazaar, if you do find them it might not be that many at a time. The easiest way is to buy them with arena tickets, I think one of he guys in the front of the arena does.