Sonia,
Im not sure where you live, but if you are in the US you can buy a pulse oximeter at REI stores or www.rei.com for about $375 (US dollars). Kind of expensive but might be worth it if you're worried about your sats dropping during exercise. I bought one cuz I like to go skiing (about 12,000 feet above sea level at the local ski resorts)... It really surprised me, cuz I've NEVER needed oxygen, even though I've been in and out of the hospital about once a year for the last 15 years (I'm now 36).....When I went skiing last year, my sats dropped to about 90% at rest at the higher altitude, and about 82-83% after skiing down some longer harder runs. Then as I rested going back up the ski lifts, my sats would take a few minutes to climb back up to about 90%. My sats on room air at home (about 4500 feet) are usually 96-97% at with rest, and 92-94% with mild to moderate exercise. When I go jogging my sats drop to around 89-90% (at 4500 feet). I'm sure if I lived at sea level, they'd be a little better.
Dave (36 w/ CF)