A woman after my own heart!! I'm into side saddle riding, but due to the fact that I live in Australia and have a difficult to fit horse (Morgan), I don't yet have my own side saddle. I'm hoping to finally have one this year.
I did, however, start my own modern habit. I found some lovely navy italian wool at a factory sale. 5 metres will do a jacket and apron, it shouldn't be too difficult to find - I don't know what cavalry twill is, but something around about a wool melton weight should be suitable. As the other ladies have said, it is all in the inter-construction.
Get yourself a good book on tailoring (
this one is my bible). I love the hand tailoring methods (pad stitching the hair canvas down etc), haven't actually tried the fusible methods yet but I will at some stage. I love sitting in front of the TV with my hand stitching! This is the ultimate as it gives you so much control. But it depends how much time you have!
For a pattern, I used the Suitability Hunt Coat and the ISSO apron pattern (supposed to be much better than the Suitabiliy one). The Hunt Coat will need to be lengthened for SS appropriate length - it should be grazing the back of the saddle. I recommend you make a toile or two for the coat - I had already made it for a regular riding coat so only needed to lengthen it and mark the cutaway - I recommend you toile it to test how the cutaway looks.
I made a couple of changes to the pattern when I made it as a riding coat. The undercollar is supposed to be cut out of felt or melton wool (I can't remember at present and I'm warm and cosy in bed...!) and is drafted without seam allowances... I added seam allowances (and altered the order of construction) to construct it like a regular collar. Also, the vent linings were drafted strangely so I changed these so that they made more sense and were easier to work with. I also made the collar a little bigger from memory, as the points were a little too small for my eye!
For the Apron, I ordered the ISSO apron pattern from
here. I haven't finished it yet (I can't hem it until I have a saddle to sit in to check the hem length and that it hangs correctly), but all indications are that it will be a good fit. I have underlined my fabric with a medium weight muslin/calico, as long as your lining colour is very close to the wool fabric, there is no reason you can't use an underlining between. Also, weight the hem using curtain weights/pennies etc to stop any flapping about. I'm fairly sure you'll find that most modern habits have a lined apron.
Hope that helps you. Feel free to ask me any questions about the two patterns or the tailoring methods - I'm definitely still learning myself, but hope that my experiences might help you!
I'll try and get a photo of the habit for you and a photo of the riding coat I made from the pattern last year as well, to give you an idea