Get in the yellow pages and call 3 or 4 companies out. Lowes has an ok Pella for what you are looking for. Home Depot has Simonton, but will be a tad high on the price. Probably @ $6.75 per UI x 272 UI = $1836
Stay away from mom and pop homemade windows. The window business is experiencing unseasonably slow times right now because the real estate market is in the shitter. Good time as any to buy...