I am interested in learning how to make them.

Are there any good online sources on how to make Lolita style top-hats and other head accessories (Such as crowns, etc...)

i suggest that you go to a hobby store and buy a premade small hat. then you can continjue your shopping for accesories you want on it. making hats from scratch is really hard for me but if you would like to do so then go to google or youtube and look for tutorials. Srry if this doesnt help... Good luck!

Hat making can be a bit of a chore unless you have the time to really invest in it. Depending on your goal and level of skill as jrocker16 suggested you might wanna just buy a pre-made hat from a craft store and build it up from there. How ever If you fully wanna give this a go, This craft site might have some info also I know Cosplay.com has a Lolita section and the members there are pretty darn helpful.
Either way good luck with them =^.^=

Yep, I believe making hats would be quite difficult without lifelong practice in the trade. Your best bet would be to buy a basic hat and decorate it as you wish. ^^ They'd still sell for a lot!