Any suggestions on the best blank 510 carts for the ego?
Yo. I think I'm one of the few remaining people who still prefers cartomizers by themselves, as opposed to tanks or clearomizers.

You're going to want to get single coil Boge cartomizers, and my personal preference is the LR (low resistance). The nice thing about Boge LR is they're about 2.3 ohm, so they're not AS low as other companies... that means you can take better advantage of the range of a variable voltage device, without being limited or having your face get blown off (...not literally).
Also, you'll probably want to get them in stainless steel (or "silver"). What this means is they won't have the black sticker around them, so they'll fit better inside an Ego cone without having the air be restricted.
The best price I've found so far is at
www.vaporschoice.com. $5.50 for a 5 pack (and coupon code vf5 should get you 5% off).
VaporsChoice.com - Boge 510D LR Empty Cartomizer
(Note: I have no idea why the product page says "Great for dripping." Ignore that. haha)