We have been using pre-cut windows in class; however, the company has become very difficult to work with and my vendor has given up trying to work with them. She found this company for me (as we use a lot yearly and since they are 8 ply can't be cut by hand).
http://www.redimat.com/

I compared prices with Light Impressions and if you want just one or two windows there isn't that much differences. However, if you start buying 12 or more the savings are serious. They have pre-determined window sizes and then they will custom cut.
I have not ordered any yet, but will be doing so in a week or so, and therefore can't vouch for quality of the board; however, my contact has another customer who has been using them and is very pleased. They too had been using Soho windows in the past and got caught in the same bind i did.
p.s. i compared the prices including my discount with light impressions.

I will get back to you all with feedback concerning quality later in the month when i have a chance to review them myself.