I had been searching for a while to get a 100x100mm IR filter. The filter available to us in the UK is the Cokin Z007 which is a 72. Lee only do an 87 as I'm sure you are already aware.
The 100x100mm Cokin Z007 is expensive, so I decided to buy a 67mm Heliopan RG695 which was about the same price from Teamwork. I was able to pick up a Hass. bay60 - 67mm adaptor for nothing on feeBay. I'm sure the Heliopan will last longer than a Cokin resin filter plus I'm getting the Schott glass.
The Kodak Wratten 89B 100mm (4in) resin filter is not available in the UK, so you would need to spend $100 plus postage and customs VAT to get that!
Here is a chart which plots all the IR filters:
all the best.