The main merchandise, and what most fans will all know about well, is the DVD of Spirit.
The DVD was released in several countries and is still available to buy online at places like Amazon.com and Play.com
It's possible you can still find it in stores but only in dedicated entertainment stores like HMV because it's quite old now.
More information here.
The Spirit soundtrack is probably the next most popular item of merchandise and was also released in several countries.
Like the DVD, the soundtrack is still available online and in large entertainment stores.
There are also a large selection of posters available on moviepostershop.com!