There are many kinds, cell sizes, thicknesses materials and dimensions of brood foundation. My favorite is no foundation:http://www.bushfarms.com/beesfoundationless.htm
If you want to regress them to small cell (instead of just usuing natural comb) you'll either want to start with a few sheets of 5.1mm and then move to 4.9mm or just go with the 4.9. The advantages are mite control.http://www.bushfarms.com/beesnaturalcell.htm
If you don't want to go to small cell, then you need to buy "medium" brood comb. Meaning it is worker sized (at least in theory though it's actually much larger) and it's of medium thickness. This would be of beeswax and you may want to wire it or at least not put it in until the bees need it so it doesn't collapse.
If you use all one size of box (I use eight frame mediums) and one size of frames (I use mediums) and you want interchangability it's nice to use all the same cell size and kind of foundation so everything is interchangable.
Another option is fully drawn plastic. Honey Super Cell is available in Deeps and PermaComb is available in mediums.
You can also go with plastic foundation. The disadvantage (from my point of view) is none of it is really the right size (4.9mm). The advantage is that it doesn't need wiring and is easy to put in. The wax moths will still eat the comb but can't eat the foundation. If you want to do crush and strain (as opposed to extracting) it doesn't work quite as well as wax.
You need the foundation to fit the frame. In wax, Walter T. Kelly has the widest variety of foundation sizes. But none are small cell. They have them specifically for most top and bottom bar and frame size combinations.
In the case of plastic it's all pretty much one size for one size frame with no allowances for other variations.