Hi C-H's,
I was ordering some lab. supplies and came upon something that some 
of you have asked for:
http://www.emsdiasum.com/
Type 65510 into their search tool to get pH test papers.
As you are aiming for a pH of 4.2 or less, the -15 or -18 would do it.
US$12.50/100
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                      Regards,               Cameron.