be easy to get a wheel from a kids mountain bike.
but its probably a 36 spoke wheel
the frame you have will likely be 110mm wide at the wheel slots on back
it needs to be widened evenly on both sides to around 130mm
I have widened frames before by lying the bike on its side on the ground and stepping on the lower drop out and pulling the top side. Some small frames are a bit strong to do that.
My prefered method now is to strip the bike down and clamp the bottom bracket, the hole in the frame that the crank goes into. In a vice. Then lever the frame wider with a scaffold pole against the seat tube.
This method is easier to get right. If you pad the tube with cloth it wont scratch the frame.
If you want a 28 spoke hub with a cassette. That'll be a racing part. Look on ebay for one. Maybe a Time trial rider selling stuff.
Alternatively. If your wheel is a plain single speed. You can fit a longer axle and redish the wheel. A 7 speed freewheel will screw on.
Be a bit complicated to do.
You might not need to weld anything to the frame. As a rear gear mech will fit to the frame with a claw bracket.
Be better if you weld cable stops to the frame. But you can run a full length cable outer.