Ebay is a good source, there are certainly plenty available there. Since you are buying for a museum and want historical accuracy, here's a couple of color guides:
http://hrd7.tripod.com/hbc/hbc.html
http://www.pointblankets.com/index.html
Bear in mind that the 4-point was very rare historically, sizes generally were 1, 1 1/2, 2, 2 1/2, 3, 3 1/2. Three and three and a half were quite common. Fours did exist (I can 'point' you to references in fur trade papers), but as noted were very rare. A four CAN be made into a three or three and a half by pulling out the point yarn, if you want a larger blanket that has the historically correct number of points.
As far as colors go, white with a dark stripe at either end was far and away the most common. Greens and reds (both with dark end stripes) were around, but not in the same quantities as the plain white blanket. Avoid the candy stripes ---multi stripes were around (in very small numbers) but the common red, green, black, yellow doesn't really match the historical blankets. See the websites above for that.
If you want new blankets, here are some sources, including a source for double blankets:
http://www.woolsoftmontana.com/
http://www.nwtrader.com/index.html
In addition, try getting ahold of Rob Stone (no website, and I don't have his contact info), he makes some of the best and most historically accurate point blankets around. Google "Rob Stone blanket" and you should find his contact info.
Probably more than you wanted to know, but I hope this helps.
Rod
