I bought a simple pattern (JoAnn's has a 99 cent sale on McCalls patterns today only). You can make her a christening dress and then put some lace on it to make it just right for her. If christening dresses are not available, just buy a dress pattern that can be lengthened.
Then I purchased some fancy white eyelet. However, that fabric is usually not available now in most stores because it is a spring/summer item). If all else fails, you can go to the bridal section and find some pretty white material.
Oh, I did buy some beautiful cotton/polyester white eyelet on sale - at least 4 yards. It even has miniature crosses at the bottom, so it would be appropriate for a Holy Mystery. If you want it, let me know how much yardage you need and then please PM me your address.
If you cannot sew, there might be a seamstress at your church.