Start off with Lowe's and Home Depot. Lowe's has a Pella Thermastar that you could do in that size w argon and low e for probably around $200. Alot of people talk trash on this window, but it's a sturdy window, and would be a huge improvement over the single pane aluminum windows you probably have now. Also check in your yellow pages under windows and look for distributors in your area. Look for an attractive fusion-welded window with a u-value of.30 or lower for best performance.
I'm not sure if you have vinyl siding on your home now, but if you do I'll give you a few tips.
To remove your existing windows take a level and pencil and mark 2 5/8" around the window on the siding from where the rough opening is.
Take a skill saw w/ a paneling blade and cut this line out which will expose the nailing fin making for easy removal of the old window.
After clearing the opening, insert your replacement window flush with the interior of your home. Set the window with 4-6 3" screws making sure window is square and plumb.
On the exterior place insulation all the way around the new unit. You can also use a flashing tape on the wall and over the gap around the window. Then place J-channel around the opening you cut out with the skill saw. Youre left with 2 1/2" between the "j" and youre window. Case out the exterior with a 1"x3" then cap off with pvc coil. Sealing where the Metal meets the "j" and the coil touches the window.
With youre window sitting flush with your interior wall you can now trim out the interior with a 2 1/4" casing and seal this as well. This is the number one improvement you can make on a mobile home. Hope this helps.