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Photo Warehouse in Southern California appears to have spooled down Double-X in stock, and they list 36-exposure single roll, 36-exposure 5-pack, and 100-foot options:
www.ultrafineonline.com
B&H also seems to sell Film Photography Project's X2 Eastman Double-X, but at a rather shocking premium of $10.99 for a 24-exposure roll:
www.bhphotovideo.com
Kodak 5222 Double-X Black & White Film 35mm ISO 250 - Kodak Black & White Films
Kodak EASTMAN DOUBLE-X Negative Film 5222 is an ISO 250 motion picture black-and-white film known for it's subtleties in tone scale and designed for general production use outdoors and in the studio, in dim light, and anywhere you need greater depth of field without increased illumination.

B&H also seems to sell Film Photography Project's X2 Eastman Double-X, but at a rather shocking premium of $10.99 for a 24-exposure roll:

Film Photography Project X2 Eastman Double-X Black and White Negative Film (35mm Roll Film, 24 Exposures)
Buy Film Photography Project X2 Eastman Double-X Black and White Negative Film (35mm Roll Film, 24 Exposures) featuring Panchromatic B&W Negative Film, Motion Picture Film, ISO 200/24° in Standard Process, Subtle Tone Scale. Review Film Photography Project Eastman Double-X