I made my own harness for my puppy when she was tiny as nothing fit her, there are lots of directions online but here's a link to my favorite instructions, took me about an hour to make the first time.
Little Dog Harness Tutorial | Dog Under My Desk
Kizzie's only 3.2lbs now at 7mos, I have a tiny step in one made for cats but it's still a little too big, we have to put it on over a sweater to make it fit ok.. I think though the home made ones were a LOT easier to put on and take off since it was just velcro