You might consider getting a second Nikon - and have the internal IR filter removed. The D200 is the one most Nikon folks use (the D700 is not suitable for IR; on the other hand the Canon 5D2 is superb!). These can be had for about $300-$400...and then the conversion for another couple hundred. However, you will find folks selling them used here, at Nikongear.com and over at FM forums.
The advantage to a converted Nikon (or Canon or Sony) is that shutter speeds are "normal" - ie., equivalent to doing the same scene with a non-converted camera - so you won't need a tripod. Putting an IR filter over the front of the lens is (a) a hassle, especially for focus; (b) slows your work method down; (c) slows shutter speeds down considerably - you will have to use a tripod for landscapes...and likely limits you to the lens you have the IR filter for. Forget about photography on the move too (so no street shooting / people photography.)
Here are some IR images from Central Park (Manhattan), with the D200 and the 5D2 (both have had their internal IR filter removed):
http://photo.net/photodb/folder?folder_id=637107double-clicking will enlarge each considerably
If you are interested in a D200 camera converted to UV and IR use, contact me, I have one for sale.
I have gone all Canon now - more Canon lenses work with the 5D2 than equivalent Nikkors (on a D200) - tough to get hot spots on the 5D2 images...this was more of a problem (though not common) with the D200 and Nikkors. (You can use your AIS Nikkors on a Canon body with an adapter...and even the new "G" Nikkors can be used on Canon bodies...with appropriate adapters. EG., the 14-24 f2.8 Nikkor is wonderful for IR, as well as standard photography - on both a Nikon IR converted body, or a Canon IR converted one - such as the 5D2.)
Finally, yes external UV/IR flashes are made - if you do a search (Google) you will find some nice evening photography using an IR converted Nikon and a modified flash (for IR). These work well for people photography (skin tones and texture are wonderful in IR.) One of the members here, Shane (beyondvisible) can make one for you at a very reasonable cost. Nikon once made a flash unit specific for IR photography but no longer does. It was quite expensive and used ones are rare (and much more expensive these days).