What are present limits on legal shorting?

[ubuy2wron]

Since we have some new members here that seem to be fairly well versed on the short side. I have a general question re reported short interest and the limits to shorting. I am trying to understand what physical limits exist to selling stock . My understanding is that shares that are held long in a clients account can be lent by a broker up to the value of any loan existing in a account and that shares held by institutional investors may be available for loan . When I see a reported short position of 20% of a companies shares which does not have a large institutional share base I wonder how can additional shares be borrowed and from what source?