Use the search bar at the top left of the Effects forum to search for 'synth'. There are approximately 5.7 million posts on synth related effects pedals.
The MicroKorg has man, many sounds in it so you're going to need to be much more specific about the sounds you want.
I've been working on a synth board and have spent thousands to get the tones I want. It's certainly a cool way to go but I'd recommend just getting a MicroKorg if you like the sounds. Take the time to learn you scales or even transpose for each different key so you only have to learn one.
Having said that, check out the following...
Digitech Bass Synth Wah
Electro Harmonix Bass Micro Synth
Chunk Systems Octavius Squeezer
Akai Deep Impact
MarkBass Super Synth
Line 6 M9 or M13
There are infinite numbers of ways you can create synth tones using combinations of individual pedals but those ones above are your best bets for dedicated synth pedals, or in the case of the the Line 6 ones, they're a multi with synth patches included.
Hope that helps a little