http://www.d20srd.org/srd/planes.htm"The plane shift spell also deposits the spellcaster on the first layer of the plane." [DMGp151 left middle]
It does NOT put you on further layers without specific, expensive material components. You can't teleport to them either because they are not related by distance: "These layers are effectively separate planes." You have to adventure through them.
The real reason for ignorance about this is that the D&D cosmology is a mess (See A Full Cosmology). Even FCI says of the largest plane: "plane shift [...] allows travel from plane to plane or layer to layer. Without the proper material component (a forket metal rod aligned to a specific layer)
all plane shifts to the Abyss deposit the caster on a a random location on the Plain of Infinite Portals" [FCIp109]. And of course, the Plain of Infinite Portals is the first layer of the Abyss. When it says 'layer to layer', it means from one plane's first layer to another plane's first layer. It does not mean from the first layer of the same plane to the second layer of the same plane. That is not possible via plane shift (see above rule quote). Indeed FCI clarifies later that if you do want to travel between the layers: "the following planar pathways offer travel between the layers" but plane shift isn't mentioned.
There are costs listed in Dungeon Magazine for specific planes if your DM is nice enough to make them available for you. But if you aren't flat out given them, there is no guarantee you can buy them and many are too expensive even to make with Wish. It looks like you'll be adventuring to layers after all.
