I picked up my spare from a junkyard ($100), but I have an E350 chassis, very common tire size, (ambulances, school buses for example). Doesn't hurt to call a junkyard and ask.
I have roadside assistance, but we have found that they are not always that dependable.
We have also encountered areas where cell phone reception is not available.
I guess if I always stay on the interstate and close to larger urban areas cell phone and roadside assistance is fine, but we do like to explore and get off the main roads sometimes.
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