I'm assuming by your comment that you found single hung as a replacement, the 30.5" is the width and 41.5" is the height. Assuming that is the case, and the sides ofthe opening are full length studs, I may have a solution that worked for me.
It requires working inside the wall a little, so it could be a little tight if you have big hands, but here goes:
What I did is expand the height of the opening by:
1) Cut the wall panels inside and out at the desired height, deep enough to cut the cripple stud(?) in the middle on the horizontal cut, if possible.
2) Knock out the sill plate, and cut off nails or screws as required.
3) Cut the cripple stud down another 1 1/2" below where the wall is cut. (an oscillating multitool is the only tool I know of that works well for this purpose).
4) Add cleats (6" length of 2x4 will suffice) to the studs on each side to support a new sill plate (I recommend driving the screw part way into each cleat before inserting it into the wall - you should only need 1 in each)
5) Install a new sill plate.
6) Buy and install a standard size window.
I hope that made sense.
Additional thought: A standard 2-6 by 3-6 window will be 29.5" by 41.5" - you might be able to, with a little 'persuasion' (or a little planing, sanding, routing or wood chiseling) get it to fit, without steps 1-5 above.