Products for senior horses are very custom to a horses needs. Some senior horses need specific minerals and vitamins others do not. Contacting your vet (even if it is just a phone call) can help you.
Smartpack also has many options for joints!
You can use a gel pad under your saddle and creams to help with any muscle or joint pain the horse may be experiencing. You can also purchase a massage brush to use on the horse after competitions. Hope this helps!