If they're only selling case lots and crates, chances are they aren't drop shipping -- or, their drop shipping options are separate from what you're seeing on the site.
Regarding your specific niches:
Avoid like the plague. You simply cannot make any money in this niche as a dropshipper. Have you seen the prices pieces fetch on eBay? Gemstones/watches/jewelry is one of the niches where eBay is definitively a wholesale/liquidation source for buyers (jewelry does better than stones, generally, but the competition is crazy.)
plus size clothing to women, childrens clothing
If you happen to live near a city of any size, look around for wholesalers in the city. In Atlanta for example, there's the ApparelMart, which has suppliers' reps from companies around the world. Most cities of any size have a wholesale market, and apparel is usually abundantly represented (plus, serious deals can be found at the cash and carry markets.)
Of course, these will for the most part not be dropshippers, but you can at least make some solid contacts who work in that niche, which can be very helpful.
possibly cookbooks or religious or spiritual items
No idea on this one, but I think I recall the religious items niche having been asked about a few times, and Satya coming up with several choices.
I would suggest that you might want to focus a little -- that's a broad range of different products to be trying to research at the same time. Frustration is a normal thing that I think everyone in this business deals with, sometimes on a regular basis. Keep the faith, and look for a single niche, a single brand, even a single item that you feel you can sell at a profit, and work on getting your first few sales. Communicate with buyers (and lookers) before and after a sale, and you'll find them coming back, as well as telling friends about you.
But you've got to decide on something to put out there, before you ever do any of that. So don't let yourself get too bogged down. Research is important, but getting overwhelmed to the point that you never really make any progress is too common a malady to newbies in this business.