It depends on what species you are looking for. Better to do a Google search on the species you want, and check out the websites. The forum sponsors at the top of this page, is one place to start. Some species are not available as CB. Trust a business that is honest about the animals they sell. (Pro Exotics rocks!)
A reputable breeder will supply you with pictures of eggs hatching. Not pics of adults, and pics of eggs, but the eggs hatching. Why? Healthy female varanids lay eggs regularly, with or without a male. Its easy to get pics of eggs, and it proves nothing, as you could be sent an adult pic, an egg pic, and some WC baby, and told it was bred and born here.
For true CBB monitor ads, check out Varanus.net, click on the classifieds. They only allow the real thing there. CB is the ideal way to start.
You could try the classifieds here on KS, but know that they don't check for authenticity. Every ad will read CB, or if its an adult animal, it will scream "proven breeder sav/nile/bt!!". Its a total joke, proven breeder means, eggs were hatched from that animal. And where are all the $200 dollar CB savs???
Riiiiiiight.......
D.