Best bet is to use the Pages Jaune (www.pagesjaunes.fr) to find a cyle dealer and then ring them up to find out if they hire stuff. Many hire as well as sell bikes. Also try the local tourist office (http://www.baiedesaintbrieuc.com).
There's a Comité Départemental de Cyclotourisme (sounds promising) at:
10 rue de Guernesey
22000 ST BRIEUC
Tel. 02 96 94 70 67
The youth hostel (auberge de jeunesse) is also listed as hiring bikes.
Hope this helps,
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