We buy the stuff Gensco (the airplane tire people) sells and it works great.  Ran it in all the tires in Africa with great success.  Son cut a sidewall on his new mower couple of weeks ago.  About a 1/4" slit on the sidewall, plugged it to slow the leak and poured some in, its been up since then.

I bought a few 5 gallon buckets of Ultraseal this spring and am very happy with the results.   I no-till into cornstalks and had all 4 tractor tires and 3 of my planter tires that had slow leaks. One of my front tractor tires had been "repaired" at the tire shop THREE times and still leaked.  I have a second planter that also had seeping tires and used it them as well.  Also had a 4 wheeler cart that always had flat tires every time I went to use it. 
I put it in all of these without fixing any of the leaks first and it sealed them ALL up.   One tire I had to add air the second day before it sealed - it just needed to be moved around more.   And I had no more new flats or low tires this year.  It really saved me a lot of grief and was my best purchase this year.

Try calling ultraseal directly for a good deal.