The towels used in the film counted as a prop and not as part of the costume. The towels were possibly purchased from a store (still investigating which one) because they wanted them to look pedestrian and ordinary, but also possibly hand-made.
Arthur's towel was white/green with a hearts pattern on it. If you have the double-DVD boxset of the film, there are some very good close-up shots of the towel in the behind-the-scenes programme on the second DVD. I haven't managed to find a similar towel in a shop yet, still looking.
Ford's towel looks like a normal purple/lilac towel. Looks very similar to the Lilac Supima Towels available from Marks and Spencer.
Very big thank you to Costume Designer Sammy Sheldon and Jamie Freeman (brother of Martin Freeman, and website guru for www.martinfreeman.com).
Estimates that there were 6 copies of Arthur's costume
The towelling robes were (allegedly) specially made from towelling imported from Turkey.