You can find some on Etsy and EBay for Kelly/Chelsea - kind of spendy though - $4-10 per dress plus a couple bucks shipping. For Skipper and Stacie, try some of the tighter-fitting Barbie items. A little ribbon as a "belt" to hold them on helps and they look really cute. If you have the time/ability, a bit of elastic or elastic thread will alter these items nicely.
Also, some of the other "fashion" doll clothes will fit. Some Monster High clothes will fit Stacie and Chelsea, Liv clothes will fit Skipper and Stacie, some Lalaloopsy will fit Chelsea, and I've had luck with some of the Winx doll items (their shoes are huge, but fit the Barbie Star Dolls, as will Liv shoes). Some of these dolls have deceptively tiny bodies (with oddly enormous heads), but again a bit of elastic helps, and the non-Mattel makers seem to put a lot more detail into their items.
Try here for some free Barbie patterns:
I don't know what sizes the dolls you mentioned are, but look in the regular pattern books that fabric stores have. They have patterns for Barbie clothes so more than likely they have patterns for the other dolls, too.
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