Buy jets at treats.
If you don't know what size you need, you can try checking the spreadsheet for people running similar setups. Although if you just threw a 15mm carb on there and haven't put on a pipe or anything, I don't know how much you're going to find that's similar to what you've got. Usually people upgrade the pipe first because you get more bang for your buck with a pipe (if that's the only mod you're making, anyway). I doubt you'll be seeing a whole lot more speed with the bigger carb unless you do some other mods too. You might want to read this.