Ditto the Lemke seats at Southwest Custom Rods. Southwest Customs carries .740" and .812" Lemke Seats for spey/ switch rods. I have purchased several. Ron is more than helpful. Plus he will custom make a beautiful insert for the hardware.
Acid Rods is also another supplier with a good selection of components.
Snake Brand Guides produces the Snake Brand Universal Guides they are the best on the market. With their concave feet they fit the radius of the blank, making wrapping the guides a breeze. More expensive than other guides, but there is no foot prep. They are ready to go out of the box and are worth the extra $$$.
As for your other questions: Cork handles come with a 1/4" hole in them. You will have to ream the cork to get it to fit the outside diameter of the blank (OD) no matter what. Most manufacturers will have the OD of the blank's butt listed. Then you can buy a seat with the inside diameter (ID) close to that. You will probably have to build up the butt with tape arbors no matter what.
Post your rod specs and you will get more advice about components than you ever wanted
Check out your local fly fishing clubs. Many offer rod building classes for a very reasonable price. Really knocks down the learning curve.
See if your blank is listed here: http://www.batsonenterprises.com/Guide-Space2.htm
May be an old chart.
Sorry for the long post, I get a bit excited about rod building