Anyone know a good source for the flat "drop free" mag retention springs?  I've heard that just hammering the OEM one flat can end me up with a broke spring.

Either the CZ-85 or -85 Combat has a drop free mag.  I can never remember which.  Find out which and you can order that spring from CZ.  It shouldn't be too tough to install if you can get the current spring in and out of the gun.

You can modify the one you have, though I don't think hammering on it is a great idea.  GENTLY bend the spring in the opposite direction that it comes bent from the factory.  Another option is to close up the loop that one of the retention pins goes through so that when you reinstall it, putting the pin through it pulls it tighter.

If you have a "retention" spring in the gun now, take it out and throw it in the trash, if it doesn't have one don't worry about it.  This feature was only put in the gun so a millitary type wouldn't drop his mag into the dirt when he ejected it. Civillians don't need or want this feature because it slows down mag changes.  100% of the guys that I knew that shot IPSC with the CZ type guns removed this spring first thing. It's not needed to make the gun function so it's just another extra part, the less parts the better.