I found some colors on the web that I'd like to use in my print brochure. What's the best way to find the CMYK equivalent of a HTML color?
Your best option is probably to talk to your printer and borrow their CMYK or spot colour Pantone® book. There’s just no quick or reliable way to convert a colour you see on screen to a CMYK or spot colour.
The Pentone books are pretty expensive, so borrowing is a good idea (or visit your printer, if you can).
If you can’t do that, then another option is to find something (anything, really) that matches the colour you like, and ask the printer to do their best to match it.
And, if you can’t do that… there are some obvious choices for strong Pantone / CMYK colours, if you want bright red, bright sky blue etc.
This may help:
Since I can't comment yet, I'll put this in an answer.
I usually use this site: Encycolorpedia
Just paste in the HEX-code and they'll give you more info on the color than you'd ever want or need.
Be aware though that color conversion isn't always straightforward, as has been said before.