I just saw that the Big Apple Ponycon announced a custom contest, too - with all gens represented!
Maaaan, people need to get moving, we need more pony conventions over here in my area!!
I hope you find a good convention to go to! Have fun either way!
But yeah, I don't think you'll find much G1 at Bronycon; I only found a couple of booths that sold toys at all (in all of the Brony-based events I've been to I mean, not just Bronycon), mostly it's art prints and the like.
And they were almost all G4.