Should not be a problem to find a pair of curtain to start with. You're going to need to measure your window and then allow some. The width is likely going to be the important part. Get whatever length they have because it's going to be shortened. Check your phone book for "alterations" or "seamstress"...something along that line or make a stop by your local fabric store...JoAnn's, Hancock, similar. You will already have a rod pocket in the top, so all you need them to do is to cut the length and put a rod pocket in the bottom. Bingo! For someone who is even moderately skilled at sewing, this should not be more than a 30 minute job and would probably not cost you more than maybe $10. Be sure you take them the measurement of the finished size you want....allow a bit above and below the window....maybe a couple of inches on both.
BTW, If you can't find a pair of curtain that's a good width, get what you like and ask the seamstress to cut them in half lengthwise to make 2 the same....then you have one to spare. Obviously a couple dollars more. OR....if you don't find a ready made curtain you like, locate a seamstress first, tell them what you want and the finished measurements and they will tell you how much fabric you'll need. Most fabric is approx 44" wide. Heavier, home decor type fabric may be 60" wide. In any event you can get this done and not spend a bundle on it.