You tend to get a lot on ebay, we used to service our own. check the bottle is in date, replace as needed, inflate them by mouth and leave inflated for a period, inspect the fabric, inspect the light battery, replace as needed, then deflate them, use a biro case and push the valve in with its edge whilst using a hoover hose to suck them flat, then repack
you should be able to find a servicing schedule online for the make of jacket.
see
1 New E.A.M LifeJacket / Vest / Aircraft Life Preserver on eBay (end time 17-Apr-10 16:37:40 BST)
Life Vest - Inflatable on eBay (end time 28-Mar-10 11:15:07 BST)
just search life jacket
Though for the safety and all that, a new one is a little cost in the scheme of things....... And I neither endorse or condone the use of the ones I have shown as an example....