I did mine entirely through the shop wizard. I found the trading post to be a rip off in the end, as I saved about 200k of what the trading post was asking for a full set. I learned that through the nice folks here in the help section!
It is tedious because searching the shop wizard for the map could bring up any piece, but if you are persistent about it, you will find deals. I got most of my pieces for 10-20k below the average asking cost. Just keep trying, and keep playing games for the cash.
Honestly, Dice-A-Roo is a big one for me. Its not flash so you are getting a lot of Random event chances for things like codestones, other map pieces, as well as the food you can sell from the game.
Good luck!