Best source to purchase water tanks

What is the best source for fresh, gray and black water tanks?  I'm trying to get the plumbing for my truck knocked out this coming weekend.  My truck is going to have a toilet onboard which is why I need the black tank.  The plans have already been approved by the state.  Looking to get at least a 35 gallon fresh water tank.  State law requires the wastewater tanks to be 15% greater.

To get your hands on tanks quick then try your local rv dealer. 

I searched on ebay and found a seller in Plymouth Indiana who would let me pick them up so I drove up to get them, to save on the shipping charge of 27.00.....I was headed to indy anyway to pick up to 100# propane tanks for 100 bucks
 
 
Water tanks were 60 bucks for 30g and 65 for 42g

We got our waste tank for free. It was tossed out at an RV park that we were staying at while in TX. Downside, it is polyethylene ( virtually impossible to repair but I have heard rumours of a glue... not found it yet). If we were to replace it, we would either build a custom holding tank or buy a Valterra ABS tank and use it for a waste tank. Since it is ABS, we could glue in fittings (and repair it) with All-purpose PVC glue (the red label stuff by Oatey that you buy at Lowes/Home Depot)

I purchased all my water supplies (tanks, pump, watter heater, fill hatch and lp valves) from ppl motorhomes.  http://www.pplmotorhomes.com/parts/rv_parts_rv_accessories.htm
They seemed to have the best prices I could find.
 
I would get the biggest tanks you could fit, having a lot of water is helpful.

The Valterra ABS tanks only go up to 36 gallons? what size is your current waste tank Lorna? you have a cart right?

Our current waste tank is 30 to 34 gallons. We only have 10 gallons ( about 8 to 9 in reality) fresh water on board. We use a tankless water heater. We top off the water tanks (two 5 gallon round drink coolers tied together) daily. I go thru more water than David does. Haven't figured out why. We dump the waste tank every day or ever other day... depends on how we feel. We just hook up a hose and it drains into the sewer connection overnight. We drilled out the hose connection (grey water connection) on our sewer elbow and put a plastic hose shut off on it. The cart hose stays on the elbow all the time and we just shut the valve off on the elbow (so it doesn't "vent" the sewer smells) and coil the hose up under the back end of the RV.

The more and more I look and rework my plan I am really tempted to mount the waste tank under the truck, the only thing that is keeping me from making this decision permanent is the cold weather and worrying that it will freeze up in the winter.

 
holding tank heater

Prevent freeze-ups in your RV holding and fresh water storage tanks. Dual voltage protects while driving and while parked. Built-in thermostat automatically activates heater when tank reaches approximately 38°F, and deactivates it when tank reaches approximately 58°F. . 1 per pack. $99.99

Oh my!!!!! how awesome are you, never knew such a thing existed, I have so so much to learn....looks like this runs on electricity, I looked at it on line but no details I will email them thank you again

It runs on 12VDC or 110VAC. You would be amazed to see what RVers  have access to.

I have a tank heater similar to this on my waste tank, runs on 110, also have the waste line wrapped with heat tape, otherwise it's one solid chunk of ice.