Breakers are not universal fit. They are specific to brand and model. You can probably find the name and model of your panel somewhere in the panel, or on a breaker. If not, you could pull a breaker and bring it to your local supply house for ID.
15 year old breakers probably aren't hard to find. (unless you have MY 15 year-old Square D Trilliant Breakers
I'm not sure why you'd replace the panel and subpanel though, if they're fairly new and in working order. Just replace the wiring in the couple circuits that you wish to upgrade to 20A, and be done with it.