As mentioned above you have to use the little stones you get from sieving dust and dirt. You can also get stones by using a hammer on smooth stone which is how I was able to setup a 6 high smeltery and a full size high oven by day 15. Please note though 1skandar has mentioned on Twitter that this functionality will be removed in 1.1.3 so that you again will be forced to scavenge parts from cities to get the smeltery and high oven going. You will still be able to get seared stone by putting cobble in a high oven.
so this way is kinda unintended?