Flubba normal batting or even fleece will be fine for place mats, and I would iron on some heavy weight interfacing too, to help keep them rigid.
The little bags are easy to make, they are an adaptation of the 'Shopper bag' in the Cath Kidston SEW book, and there are plenty of free tutorials online for very similar bags, or I could attempts to explain it to you?
I use magneitc clasps in most of my bags and wallets, they are really really easy to attach, all you need is a seam ripper to cut two tiny slots in your fabric, the magnetic clasps come with a washer that you attach to the inside of the clasp once it is through the fabric, then you just push the two prongs down (I use my fingers but if you are not strong you can use a screqdriver, pencil, or the edge of a table to puch them down).
I get mine here. Affordable, free p&p, really quick delivery.They also show you insert a clasp, but use a chisel, you seriously don't need one, a seam ripper will do.