We've often done the 'do it yourself' princes thing. We actually prefer it.
Seriously, why waste WDW vacation getting your nails done? there's too many fund things to do once you are at WDW. Get your nails done BEFORE you go to WDW (and maybe hair?) Maybe ask the manicuris to add some fun little design (A simple Mickey? rainbow colors? glitter polish?)
Our extended family has done the barber shop thing once, and that is worth doing, but after once we do our own. ALL the women spend time getting ready for one nice dinner while we are at WDW. Instead of being a one-girl thing, it can be a fun group 'ladies' bonding time.
You can get glitter hairspray at any costume place. We call it 'pixie dust.' Usually this is easiest to find at Halloween, but you can find it at many places even now- like a party store or big chain drug store, maybe even a big box store or dollar store. (It's $.99 a can!)
You can get Disney costumes year round now. Look online if nothing else is an option, but many big box stores also carry them year round. We also have some simple DIY options. You don't have to go 'full princess' to have fun. I'll even caution- many costumes are MISERABLE to weat at WDW in the summer heat! Crinline is cruel in 90 degrees. I've also seen some evil homemade costumes too that were simply too much clothing to wear in summer. Simple and practical is often best.
Next, you can get great temp tattoos at any place with Disney party supplies. You can also find bracelets, etc.
You can top off the hair accessories, costumes, etc, with a trip to DTD. WoD has princess stuff galore. A simple little hair decor can do magic. Again, you can also get lots of fun stuff before you go. Michael's crafts had amazing 'stick on' glittler ribbon. You could so easily decorate any hairdo with fun ribbons.