I do not know what size you have, but here are a few links to holding tanks. See if these work for you.
IconDirect. I have bought from these folks before and had good service and reasonable prices for the higher quality they provide. RV Waste Holding Tanks | Replacement RV Waste Take | Grey Water, Black Water Tank
Tweety's. I have bought from them before and had good service Tweetys.com - RV Parts & Accessories - RV Hitches - Black and Grey Water Holding Tanks
Dryers. I have bought from them before and had good service RV Permanent Holding Tank
Ameri-Kart. I know of them, never bought from them Ameri-Kart - Recreational Vehicles (RV) Parts and Tanks | Ameri-Kart
Pelland Enterprises. I have bought from them with good service. They have a lot of inventory. They may not be the cheapest but what bought was good quality. - RV Windows
And it seems Amazon is in on this too. Just have to search for them. Try a hit on "RV holding tank"
Hope this helps
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
Google Custom Search For Sunline Owners Club