I have a couple S2S kayak carts as does a friend and with the inflatable tires, they go through sand and rough terain well.
BUT…
No one sells replacement inner tubes!
S2S does not carry them. REI who sells the cart does not carry them.
no one does.
You would think tha tthe company who makes the cart will also provide replacement parts for repair but they don’t.
So when Harry got a flat and tore the valve loose, his cart was, effectivly, an $85 piece of trash!
Does anyone have an idea of where to buy these things?
Amazon sells them for a very high price plus shipping.
I had the same issue
with my Paddleboy Nemo tires.
A lawn mower shop had the right size.
Check hardware stores
I don’t know about tubes for those tires, but Ace Hardware sold several types of small wheels for carts (8" to 10"). Worth taking a look.
Replacement wheels
Try this link for Wheeleez. Maybe you can find the size you need.
http://www.wheeleez.com/utility-wheels.php
Small-Engine Shop
– Last Updated: Jul-18-12 3:10 PM EST –
Riding lawn mowers have tires in a great variety of sizes, and inner tubes are available for all of them. The same is true for the tires of small trailers and wheelbarrows. If the tire size isn't provided on the sides of the tires of your cart, just measure rim diameter and distance from rim to tread. Tire sizes for small machines are expressed this way in inches, and often there is a bit of a range in tires sizes for which a particular inner tube will fit. I can't recall if knowing the width of the tire is ever necessary, but you'll figure that out on your own, because it's all very simple and self-explanatory once you start looking at what sizes of inner tubes are available.
Harbor Freight may have them.
They have a pretty wide variety of utility wheels.
I tried
Ace, Home Dopt and Harbor Freight. no luck.
I keep the tube size in my wallet jsut in case
3.60 x 4 & 4.10/3.50-4
which seems strange as they are both S2S carts but both tires are differnt size with a differnt designation.
I’ll look for a lawn mower place and perhaps a wheelchair store.
Northern Tool
Northern Tool has 410/350-4 tubes
http://www.northerntool.com/shop/tools/category_tires-wheels+replacement-inner-tubes+4294944221?cm_ven=natural&cm_cat=netconcepts&cm_pla=Google&cm_ite=inner%2520tubes
Regards,
Goobs 
Jacks Small Engines is cheaper
http://www.jackssmallengines.com/tubes.cfm
Make sure you get the correct style valve to fit the rim.
Regards,
Goobs