I came very close to getting an infrared filter recently, and finally decided to skip it for now and got a neutral density filter instead. (Totally different application, but still satisfied the "new toy" urge.

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It sounds like, to get the best results without modifying your camera, it's like shooting a photo at night - you'll want a tripod, and metering and focusing will also be tricky (the latter especially, since you won't be able to see through the viewfinder! This is where a point-n-shoot or DSLR with live view will have an advantage.)
If you do want a filter, I'd recommend checking out eBay - I've bought a couple filters now from Spotlight Photo (slphotogear), they have very good prices on Hoya (and other) filters and $3 shipping, and they ship very quickly. There were also some no-name infrared filters - I'd almost be willing to try one of those, if I was just dabbling with infrared.