1) Welcome to iRV2, and hope to see you around a lot more.
2) you're looking for a truck brake parts house. That might also be a place w/service bays doing various truck work likes brakes & clutches. These outfits usually have bins of the refurb parts as well as pre-rebuilt and new parts. By clicking on your name & then "View Public Profile" I see you are in the Junction City, OR area. You can add this and your year/length/model of coach to your signature for easy reference for folks willing to suggest answers to questions here. There are numerous service outfits in the JC area who, if they don't sell parts direct can give you a referral. In the greater Sacramento, CA area I'd send you to Capitol Clutch & Brake in West Sac, but for JC OR, I'll defer to folks more local to that area.
3) I'm assuming you are referring to the air cylinder that actuates the park brake (I'd call the air reservoir ahead of and above the rear axle the air "can"). If the cylinder actuator isn't what you are having the issue with, disregard this post.
Baja-tested '08 2-slide 36'
Alpine: The Ultimate DIY'er Project