I know that the claims state that Oleander is poisonous, but can it still be purchased as an herb?

Yes, oleander is extremely poisonous.  All part of the plant are deadly, even the honey from it.  For this reason I have never seen the raw herb sold.

And due to safety issues and a lack of proven effectiveness I recommend avoiding it.  There has been a lot of hype surrounding, but no real evidence of effectiveness.  In fact the only study that could be found on effectiveness was done for cancer and it was found to be ineffective.  I have posted on the safety, hype and misrepresentations about oleander a number of times:

http://curezone.com/forums/fm.asp?i=1796093#i

http://curezone.com/forums/fm.asp?i=1796559#i

http://curezone.com/forums/fm.asp?i=1834368#i

http://curezone.com/forums/fm.asp?i=1724192#i

http://curezone.com/forums/fm.asp?i=1553709#i

http://curezone.com/forums/fm.asp?i=1715317#i

http://curezone.com/forums/fm.asp?i=1705502#i

Bottom line is that the only substance in oleander soup that has been shown to exist in any level possible of having any significant effect are the polysaccharides.  But polysaccharide sources are found in so many sources that do not have to be specially processed to render them non-toxic.  And many of these herbs can address other aspects of diseases, which is very important.  Polysaccharides primarily work by increasing white blood cell activity.  But this is not always desirable to begin with, such as with autoimmune disorders.  But in cases of let's say cancer then polysaccharides can help to a limited extent.  If someone really wants to address cancer though they should address the various aspects of cancer that can be targeted as weak points in these cells.  For example addressing cancer microbes,  increasing interferon and other cytokines, increasing peroxides, addressing the Cori cycle, blocking angiogenesis, blocking hyaluronidase to prevent metastases, etc.  Simply stimulating white blood cells with polysaccharides alone is going to do virtually nothing for cancer as the studies have shown.

As an example myrrh gum is rich in immune stimulating polysaccharides.  It is also an excellent antimicrobial to kill cancer microbes, and it helps prevent metastases by blocking the enzyme hyaluronidase.

I've read that it is very good at raising white blood cell counts. Is this true?

No, this is not true at all.  The polysaccharides from oleander stimulate white blood cell activity, but they do not increase white blood cell counts.  Low white blood cell counts are most often from bone marrow damage.  This can be from a number of things including nonsteroidal anti-inflammatory drugs (NSAIDs), chemotherapy drugs, radiation, certain cancers or infections, etc.  If you need to raise your white blood cell counts the best herbs for this are suma, turkey tail mushrooms and astragalus root.