For a standard flight computer Pooleys do one with interchangeable inserts, so it'll go down to about 40 knots flying speed.
For a simple wind-vector diagram that you can use for any speed and nothing else, Flylight's A3 flexwing kneeboard incorporates one, if you don't fly flexwings, you could always ask them (01604-494459) if they'll just sell you the insert or do an A4 or A5 version with a similar insert?
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