I'm using a cheapo $10 pc cooler fan from Amazon that has been pretty good. I highly recommend a scythe 100x12mm fan. The recently discontinued their 2000 RPM version, but I was able to find a few Shuriken Rev b. coolers on FrozenCPU that use a PWM(!) version of the fan for $29 apiece and ordered two of them. I figured I've already spent a small fortune on noctuas, so a whole cooler for the same price as a single fan wasn't that big a deal. I'm using one on the AXP 100 I smooshed into my Dan A4 and will put the other on the K39 once it shows up. the 1000 rpm version of the Kaze Jyu slim (same dimensions) will run you $12 or so and is available on a number of retailers.
You could also try the Thermalright TY-100. It's way more performant, but sounds like a hovercraft. I like my pcs to be as quiet as possible, so I haven't used it.
The 100m is cool because it has the same mounting holes as the 92mm, doesn't require VLP RAM (I'm using Corsair LPX), and have so far yielded better cooling performance on the black ridge and an l9a (better coverage for the slightly larger heatsink) and should do so on anything with a larger than 92mm surface area ( c7g and AXP 90 have this kind of heatsinks for reference).
EDIT: I tested out the Ty-100 and it's been amazing. Same temps as the pc cooler fan, but with silent mode enabled in the bios its been way quieter