Looks like others have similar problems. check out the advice here:
http://www.supercheats.com/gameboyadvance/questions/pokemonruby/50668/how-do-i-get-the-devon-scope-i.htm
or on this page of questions you can search for 'devon' and find other answers.
http://www.supercheats.com/gameboyadvance/pokemonrubyquestions.php
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