Try Etsy.com You’ll find lots of vendors who specialize in Regency wear.

Chiffon is a modern fabric but produced mainly in polyester. For a true historical look go for silk chiffon.

And I really didn’t see anything wrong with the bodiced petticoat under your dress. It’s ok for it to not be *exactly* the same lines as your dress. It’s an undergarment and should be cut smaller/more narrow.

As for the chemise, it should be worn under your bodiced petticoat, as it would under a corset, to help protect and keep your supportive garment clean. So the layers: chemise, bodiced petticoat, any other petticoats then your gown.

The chemise at this time hits about knee to calf level and is pretty plain – no fancy embroidery or hand work. Your petticoat hem should be just a couple inches above your dress hem. You can always add a straight panel to the petticoat hem if you need to lengthen it.