I might be able to help some. I do a good bit with polymer clay, but I'll tell you up front that I haven't made bow centers. Mostly sculptures. But I did recently make some flat heart ornaments to hang on a little tree for valentines day. You have lots and lots of options with polymer clay. Two of my favorite places to shop for it & anything related to it are:
ooakartistemporium.com & clayalley.com
I've purchased from both of them plenty of times and they both give awesome customer service too!
As far as how to make things, if you want to make a lot of the same thing that has some details on it, it would probably be faster for you to use a mold. I prefer silicone molds because the clay pops out of them super easy & you can even bake the clay in it if it is really detailed. There are lots of silicone molds on Etsy. You can also make your own silicone mold of something that you created or a favorite button or whatever. If you are going to do that, I would get Alley Goop from Clay Alley. That stuff is AWESOME!! Let it set overnight though before you remove what you are taking the mold of, it just works better that way & it comes out clean.
As far as shape cutters, your best bet is using metal cookie type cutters. Clay Alley has a good selection of them. Or you can go to polyclayplay.com & click on products. They have LOTS of Kemper Cutters, which come in all kinds of shapes and varying sizes. I have a bunch of them and they are awesome. They have a little plunger thing inside of them so the clay will come out and not get stuck inside. Some are quite teeny, so if you wanted a teeny heart to use as the nose of something or little circles for eyes.... there you go. To get your clay ready for using the cutters, the easiest thing to do is roll it out with a pasta machine that is dedicated to clay only, but if you don't have one, you can roll it out with an acrylic roller.
You can decorate your clay with all kinds of beautiful things too. Check out ooakartistemporium.com for lots of those things. She carries microbeads & ultrafine glitter plus the adhesive products that you can use safely on the clay.
Idk what else you might need to know, but hopefully that gives you a good start.
As far as where to purchase clay, it just depends on the brand and what size package you want. I've bought clay from ooakartistemporium.com, clayalley.com & dickblick.com. At Blick, their shipping is a little on the high side, so if you aren't buying a bunch at once, you'll want to see how much you are saving with them and compare it to how much it is somewhere else. You can get clay from the craft stores too, but I know the ladies at ooakartistemporium & clayalley have fresh clay, so if I can buy from them I do. But I've also purchased from Hobby Lobby & Joann's before when they had a really good sale on the small packages.