Skirts should hit above the ankles – remember lighter colors are cooler.
Deseret Industries is a great place to get skirts and shirts. There are racks of long sleeve button up shirts and lots of long, full skirts.
1 or 2 bonnets or 1 wide brimmed straw hat with chin strap (a nice length of ribbon can be added for this).
A list of places to get bonnets: online retailers, Deseret Book, temple dress stores, etc. Deseret Book or Mormon Handicraft probably have the best price at $12.95. Making one is also a great option, if your up for it! Patterns are available at your local fabric store, but make sure and wait until they go on sale for a dollar or two.
Bloomers can be made or bought. One idea is a pair of scrubs that a casing is added with ribbon or elastic.
A pair of light weight pajama bottoms can work the same way. These are to be just above the ankle or the same length as the skirt.