SissyLouisa (the only one so far to leave a comment) has The Dirty Mind of a Wimpy Sissy, which has an abundance of forced feminization - and even more BBC worship (that is 'Big Black Cock' for those out of the loop) captions. But As a sissy I don't think you should be picky about color or size :D
Michelle's Feminine Place Caps has an assortment of forced feminization, 'caught dressing', and persuasive crossdressers captions.
Club Sissy has a section dedicated to captions from a variety of authors (I know Michelle above is one of those authors, for instance). The content can be hit or miss, but there's surely something for any sissy. Unfortunately Club Sissy does require registration to view the caption library.
Sissy Kiss has a "captioned piccies" gallery of user-made content. Sissy Kiss' audience favors ABDL (that is Adult Baby/Diaper Lover), so most of their captions trend towards incontinence, being forced to wear diapers (although often with frilly pink dresses over them) or girls who are still young at heart.
And to end today, here is one of my favorite captions. Have fun, sissies!
(I'll also point out that neither of the captions in this post are mine; I'd hate to imply credit for other people's stuff)