I always create inner corsets for my strapless and more structured gowns. I'm pretty terrible at explaining but I shall have a go. I have learnt through research and trial and error to develop an inner corset method that I like to use so this may not be exactly how it 'should' be done but I find that it works really well. I've also worked in a couple of couture bridal shops where they use very similar techniques to this:
An inner corset is sandwiched between your self fabric and lining. They are typically made out of a twill weave fabric called coutil and have boning. I like to use spiral boning as it gives great support but is also flexible allowing the wearer to move. I create boning channels from the seam allowance and insert the boning. With strapless gowns I assemble the self fabric, lining and inner corset separately and then attach them at the neckline and zip. You can also tack the coutil onto your self fabric and then assemble the self fabric and coutil as one attaching the lining separately as you would if there was no inner corset involved.
With an inner corset you can either create the same structure as your self fabric or have a much more contoured structure incorporating underwires, cups, overwires etc. I don't use commercial patterns so I don't know of any to recommend but it should be relatively simple to draft an inner bodice pattern from a princess seamed commercial pattern using the first aforementioned method. This would involve tracing the bodice section of your pattern pieces down to the hip section and using this as your pattern for the inner corset.
I like to use inner corsets to enhance the figure so I also incorporate coutil panels with eyelets into the side back seams 9you could also use the side seams) so that they can be tightly laced up before the back fastening is closed. This will only really work if you are making a garment with a centre back fastening and side back or side seams (most commercial patterns will have these). I draft my lacing panels to be about 6cm lower than the back of a strapless gown and about half the width of the side back panels (approximately 4cm wide). I then sew these panels into the inner corset structure by sandwiching them between the side back and back panels and constructing the side back seam. The panels should be on the inside of the corset structure so that they can be laced up underneath the back fastening. This is how the structure would look with the panels before the lining and back fastening is inserted:
If you are incorporating lacing panels then the lining will need to allow the panels to come through to the inside of the garment. I do this by constructing the lining as normal until I come to the side back seams. For these seams I construct the seam above and below where the panel will come through leaving the length of the panel un-seamed. I then attach the lining to the inner corset and self fabric at the neckline so that when it is turned in the right way the coutil will be sandwiched between the self fabric and lining. I then pull the lacing panels through the gaps in the lining and hand stitch the lining around the panels to secure it. Finally I finish off the garment with the back fastening.
I hope this helps! Let me know if you have any questions or if you would like me to clarify anything.