Su is the sodium salt of usnic acid. Essentially its lacking one or more of the acidic hydrogens. While some people claim SU to be a more bioavailble form of UA, neither the literature, nor feedback can really support this claim. Users generally end up using similar dosages of UA and SU. Par Deus ran SU as high as 2g+ on his cycle so clearly SU couldn't be THAT much more bioavailable. The only difference that I can really comment on is price. $25 for 30G of UA is by far the best deal on the net.
As for a transdermal, it wouldn't deliver any localized effect because of its need to get into the mitochondria to uncouple oxidative phosphorylation. You could put some UA in a ph gel and go for a formula that delivers it straight into the bloodstream, but it probably wouldn't deliver any more results than just taking it orally.
Stock on UA should be coming within a few days...
Thanks
WYD