There's not anyone selling complete build kits as far as I know, but you can order a complete group set- BB, crank, derailer(s), shifter(s), and brakes from online shops. Then you'll need to get wheels, seat post, seat, stem, bars, grips, and pedals. And maybe a couple of other things I'm forgetting right now.
Reading the build threads here will show you lists of all the parts you need, and give you an idea of all the options. The biggest thing is making sure everything fits and is compatible- bottom bracket style (I suggest BSA), seat post diameter, front derailer mount style (unless you are doing 1X), and axle style/size are the big ones.