Chrs,
Im certainly no expert but;
You can make knife handles out of most anything, generally antler bone and wood are used with spacers of birch bark of leather if they are what i call compound handles (dont know the propper term), ie blocks of different things stuck (epoxy resin) to the tang and then shaped - these are what i have made a few of.
You can also experiment with the grain directions to get different effects so dont feel you have to constrain yourself to grain following the line of the tang.
Obviously softwoods are softer and may get dinked more easily hard woods are harder to shape but have better longevity.
I have bits i have salvaged and let dry (oak, birch and cherry) some old fence posts, some spalted birch - damaged wood can have interesting patterns.
Bufallo horn looks good as does antler, particularly at top and bottom.
Ive used a few Helle blades and they are rather nice but often very broad and make slightly bruteish knives - not for whittling, although this does depend on the type of course!
I dont have a huge stockpile but am hapy to share bits around - i do have some spare epoxy resin i bought a few in a good deal.
Feel free to let me know if your anywhere near cambridge I can bring some bits for you to see. Ah minesota, i see that may be less usefull. lol
Also i have heard of very dense woods coming from roots - never tried it but this may be interesting. If you cut soemthing green cut it down to blocks and make sure it has shrunk completely before using!
Also search the forums here there are lots of how to's