Today there is a large variety of women's underwear in most Target or Big W stores in Australia or if you live in the US or Canada, Walmart is a magic place to browse. Just be sure that you have her right size imprinted on your mind and that you allow plenty of time to choose. Even in these less expensive stores, stockings, bras and panties still cost money and you don't want to waste any by buying the wrong size.
The best way to find out what size your Lady wears is simply to casually ask her. If that doesn't get you the answer, then check the washing on the line or in the dryer. If that fails, then you may need to do a panty raid and check in her lingerie drawer to get the right size. While checking for size, take note also of the styles she seems to have the most of because these are her preferences.
Does she prefer a thong or a high leg? Check to see if she mostly wears bikini style or does she prefer the full brief? Everyday wear many women prefer to wear a full brief or a high cut pant leg because they seem to be the more comfortable cut and the fabric is stretchable for sitting down, getting up or running around.
Whichever style you choose, just be sure to only get a pair of panties that has a cotton gusset. This cotton gusset is essential for feminine hygiene. Also, make sure that the package hasn't been opened and it is still all intact just to be on the safe-side.
If you are buying her some everyday panties as Christmas stocking fillers then a good buy can be found in the multi-packs. These multi-packs of women's briefs come in an assortment of colours and once you know whether you need the full brief, high cuts or bikinis, they seem to be the better buy. And the Bikinis aren't all that un-sexy but they are eminently practical.
If you decide that you can splurge a bit, why not get both some everyday underwear and a pair of really special knickers with matching bra for just those special occasions when the panties don't usually stay on very long anyway? This then becomes a multi-functional gift that lasts for many special times through the year.
Women's Underwear as a Christmas stocking filler can be practical and yet a nice gift to receive.