Currently I'm using those wire shelves other's have mentioned. They look fairly nice and are sturdy. The one drawback is the wires either not lining up all the way with the feet, leaving impressions in them, or bending under the feet if it's a heavy amp. My solution was to cut some 1/8 inch luan plywood to size and lay it on top. Works perfectly.
While the wire shelves do work, I have limited space and want/need something where I can fully customize the size so everything fits better. The 36 inch wide shelves are too big to fit multiple side by side in my space. The 30 inch ones aren't wide enough for all my amps. And the shelves can only be adjusted in 1 inch increments which would either line up as not enough space for the amp or not enough room for one more shelf at the top. For most people these shelves will work just fine, but I'm trying to set my stuff up with as little wasted space a possible.
I am planning to build some custom shelving for all my amps in the near future. I'm going to use black structural pipe and hardwood shelves. There's a bunch of different types of brackets for everything so I'll be able to make it the exact width and height I want. I'll also be able to set the shelves just right. I'll be able to optimize the space I have with nothing wasted.