Where do I find a replacement tricycle seat for a 10" trike?

It is not possible to provide guidance for such a generalized request. Try a Google search for "assemble (your brand/model) trike". If you can't find a good video, or if it's a child's trike and you have to ask you should have someone else do it.

I’m happy to help further over the phone at https://www.6ya.com/expert/donald_f2ed37026a3ac881

You should be able to get it from RadioFlyer. According to the RadioFlyer website you would have to call to order the part.
1-800-621-7613, Monday - Friday 9:00am CST - 4:00pm CST.
The link below is the parts page with the trike on it.
http://www.radioflyer.com/trikes.html?p=2&ref=13

Really? I just Googled "10x2 trike inner tube" (link below) and find lots of sources, including Walmart and Target.
10x2 inner tube Google Search

I’m happy to help further over the phone at https://www.6ya.com/expert/donald_f2ed37026a3ac881

Try calling - 800 - 626 - 2811. They helped me with hard to find parts for my tricycle. Very helpful and very inexpensive!

this ia question - does anyone know hoe to remove the pedals from a roadmaster tricycle ?

I've been in the same boat. I have an older model Roadmaster tricycle I am trying to fix up for my son. I found out Roadmaster went out of business. Their products and service was bought by a company called Pacific-Cycle. They do not have a lot online but yo can call them at 1-800-626-2811. They were very helpful. They are going to e-mail me a diagram and parts list. I know they have the metal replacement seats. Not sure if they have the plastic ones. Hope that helps

Try to search on amazon. here: Give your little one the ultimate holiday gift. Hope this was helpful. cheers

go to cycle shop

You need to take the old seatpost from the trike to a good bicycle dealer and tell them you need a seat to fit it. Most good dealers have parts for childrens trikes and bikes or can order them for you.

I need a new seat