I've found that sigma's products work really well for both FLAG and MYC tagged proteins.

For example:

http://www.sigmaaldrich.com/catalog/product/sigma/ip0020?lang=en®ion=HK

Hi there,

Any 9E10 antibody derivative will do... Personally I use it from Santa-Cruz.

I used 9E10 as well!

Thank you!

You can try this kit:

http://www.piercenet.com/product/c-myc-tag-ip-co-ip-kits

Moreover, I would suggest to use PLA assay for the validation of your CoIP results.

Best wishes!

Sigma's

At ChromoTek you can get a free test sample and see for yourself:

http://www.chromotek.com/products/nano-traps/myc-trapr/myc-trapr-a/