Lots of options here:
http://www.seatingsolutions.com/
And here's some info on pricing:
To buy elevated bleachers [4] with first-row seating on a raised platform typically costs $90-$150 per seat. Indiana-based Century Industries[5] sells elevated bleachers seating 300 people for $37,500, at a price of $125 per seat. The advantage of elevated bleachers is a heightened view of the playing field or event.
To buy bleachers seating 15-100 people typically costs $645-$9,000 including delivery. Bleachers seating 450 people can cost up to $56,000. This does not include the very high cost of installing bleachers in school stadiums, fairgrounds and professional sports arenas, which ranges from $150,000 for used bleachers to more than $1 million for new stadium seating.
http://activities.costhelper.com/bleacher.html
So for roughly $200,000 we could add around 2000 seats on the east grand stand ($56,000 per 450 sounds like the high end of the spectrum, you'd still have installation fees but you could also make a little dough selling the old bleachers). That gets capacity to around 10,700. Good enough to accept some growth for non-rival games and playoffs. Or spend $400,000 and get to 12,700, anticipating stronger demand and eliminate cost of bringing in temp seating for Montana (more savings).
The university is constantly under construction, and spending money (100's of thousands) on projects right and left. These numbers hardly seem impossible. But it obviously requires the backing of the administration and BoT.
An administration with vision recognizes the immense amount of publicity and marketing benefits that the football program has brought to the school over the last decade. The value has been in the millions. We all know that.
As Dopa suggested, this type of money should be within the facilities budget anyway so perhaps they are closer than we think with the Gateway and it's worth waiting for.
Or maybe we need to start a crowdfunding campaign and get the ball rolling. I'd throw money at it and recruit people to chip in.
edit: my capacity numbers are a bit off as you'd be replacing existing bleachers and I'm not sure how many they seat, but the general idea doesn't change.