Hey Chloe
Marc has a good point that you should try to find a printer who is comfortable doing spot color simulation for you. They will probably get as close to the visual appearance of the actual spot as they can, which is going to be your best bet for getting a particular color.
The only really reliable way to do it is to have some kind of proofing which actually represents how a given printer will print. If they say they can match GRACoL, you not only need to convert the spot to process using the GRACoL profile (GRACoL2006_coated1, available at gracol.org), you need some way of seeing how that will look. Every set of CMYK numbers will look different when printed under different conditions, and as Marc said, the Bridge book is only useful for printers who print under the exact conditions that the book was printed under.
If you calibrate and profile your monitor, you can get a fairly accurate representation using the soft-proof function of Photoshop. Although a screen will never look EXACTLY like a printed sheet, it can be quite close. The other option is get or make a "certified" proof that has been output to match a print condition like GRACoL or SWOP_coated3 (or whatever the printer says they print to). Once you get a reliable proofing setup, you can just use the Pantone process guide as a starting point, then try variations on that formula until you get the color you want. Most Pantone colors can't be reproduced with CMYK inks anyway, so it is more about getting the color that works for you than getting something you see in a swatch book.
The key to this working is that the printer must be able to accurately and repeatably print to a certain visual target. If they are not consistent, you will never be able to get the same color twice. If you can't afford to create or purchase an accurate proof, perhaps one of the printers can supply you with a swatch book of how certain CMYK combinations will print. Or if they are cheap enough, you you could have them print a swatch book that you create!
Good Luck!
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