In general you should bear in mind that my main interest and research is in the British mid 17th century clothing, this is my hobby (I have a completely unrelated day job) and I do not make clothes or patterns for commercial purposes. I will endeavour to help anyone with their questions, but I may not manage to reply straight away.
Either try the link on my homepage to the Elizabethan corsets or try the books listed in my useful book list, especially:
Corsets and Crinoline
By Norah Waugh
Published by Batsford
ISBN 0 7134 5699
The Cut of Women's Clothes 1600 - 1930
By Norah Waugh
Published by Faber & Faber
ISBN 0571 085946
Period Costume for Stage & Screen - Patterns for Women's Dress 1500-1800
By Jean Hunnisett
Published by Players Press, Inc.
ISBN 0-88734-610-3
Patterns of Fashion, The Cut and Construction of Clothes for Men and
Women c. 1560 - 1620
By Janet Arnold
Published by Macmillian
ISBN 0333 382845
Try the books listed in my useful book list or look at contemporary paintings from the period.
I have spent many years researching 17th Century costume, if you want to research the lives of 16th and 17th Century women I recommend you start with the following books:
The Weaker Vessel
by Antonia Fraser
Published by George Wiedenfeld & Nicholson Ltd, London
ISBN 0 297 78381 5
The English Family 1450 - 1700
by Ralph A Houlbrooke
Published by
ISBN
Women all on fire - the women of the English Civil War
by Alison Plowden
Published by
ISBN
I am based in the UK and find ridgeline boning in habadashery departments of department stores or in fabric shops. It is usually hidden with bits and pieces like velcro and pipe cord.
I have assumed you have tried ridgeline on a word search. (or is it ridgoline?). Try needlework supplies as well.
Julie Yoder from the Clann Tarten advises that Ridgeline is available in the U.S. from Greenberg and Hammer, Inc., 24 W. 57th St., New York, NY. They apparently carry Rigelene, the endcaps, steel boning by the cut piece or by the pound, and thousands of other sewing necessities.
Amount of boning? Now that's a question. As I have a wooden Busk which I can use centrefront I don't need that much to stiffen. Following patterns of historic stays I find that a bone every half inch works for the front and back. I usually put a few lines of boning at the side seams as well. It depends how much support you need/want, how much stiffening you've got and a basic 'feel' for the stays.
Metal boning would probably be better then plastic boning, if you can cut it safely. If you can get ridgeline-end caps the ends would be safer and more comfortable, but if you can't get ridgeline, the end-caps will be equally hard to come by.:-)
If you are a re-enactor in the UK you will usually have no problem getting clothing to suit most pockets and tastes from a number of traders who are to be found at larger re-enactments and at a number of re-enactors fairs through out the year. Caveat Emptor - buyer beware, please do a little research before you buy. If you live elsewhere I am afraid that I cannot help you.
I don't do patterns, I scale up original patterns from the books noted in my useful book list. I've had a look at commercial patterns which might be of use to you. Many fashion houses do have 'fancy dress', especially in Autumn for Halloween and also for Thanksgiving. Available at fabric shops or haberdashery shops. One I looked at which might be suitable for a theatrical production is Butterick no.6305. It might be numbered differently in America. It is no good for re-enactment but I hope you find it useful.