Welcome to the forum.
You question about vendors is the most frequently asked question from new members/builders. The answer is, there is no answer. You'll get many opinions from others who have had either good luck or bad with certain kit sellers. We usually suggest to look for one with good after sale support/warranty.
You'll see cheaper kits on ebay than from some of our sponsors here but your customer support might leave you dissatisfied.
As far as the kits, they are pretty much the same no matter where you buy it. They are all Chinese, if we're talking 2 stroke in-frame kits. You didn't state your engine choice but the so called, HT, China Girl 2 stroke engines are by far the most common.
You also might explore 4 stroke kits or even electric power, which is growing in popularity.
Good luck.
Tom