Why don't you just make them out of rice paper and non toxic markers? I have rice paper butterflies on one of the cakes in my photos. Look at the bassinet cake (butterfly baby). I found really good instructions on another site. CC won't let me post the site name so if you are interested PM me and I will send you my email so i can email you the link.
Oh yes rice paper b'flies are soooooooo pretty if you take the time to detail them with the non-toxic markers.
It's a bit tricky to work w/rice paper but the results are soooo stricking.
As far as I know Earlene Moore is the originator of this techinque some 20 -30 yrs ago 
She might still have the instructions on her site.
Also, I have run rice paper thru my (special, dedicated) printer, making perfect copies of actual b'flies I found on the internet and in books. Do be careful though - I had too printers and one worked just fine but the other would gum up w/rice paper. Oh what a mess that was to clean up! 
If you don't want to do the rice paper thing and still want a mold, I found a really good (and cheap) butterfly mold in the scrapbook section at Michaels. It's supposed to be used as a stamp, but it works great as a mold. It has several styles of butterflies on one sheet. I did quite a few butterflies for one of my cakes...made them out of gum paste because I can get that pretty thin and it dries quickly. You can see it in my photos. I didn't paint them realistically...just colored fondant with pearl dust and disco dust to make them shine.
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