First off, I think you are REQUIRED to have a letter for a podiatrist.
Barry: One (1) letter of recommendation from a Doctor of Podiatric Medicine.
Kent State: A letter of recommendation from a podiatrist or other medical professional
Temple: An academic letter of evaluation is required from an advisor, premedical advisory committee or two letters from science faculty and ONE letter from a podiatric physician.
Scholl: One (1) letter of recommendation from a Doctor of Podiatric Medicine.
Second, if you shadow a D.O. or M.D. and assume that a podiatrist will be extremely similar you will be surprised. You may not think that you will like the occupation if you shadow a podiatrist for a couple, so there is always that. I was lucky because the podiatrist I shadowed is a family friend, I use to do landscape for him throughout HS and some of college during the summer, and he was my podiatrist when I had Plantar fasciitis. Then the other two podiatrist I am already planning on talking to during my off year includes a podiatrist my cousin who is at D.O. school has shadowed as well and he told her to me give him a call. The other podiatrist worked in the same office as my family physician.
You mention that you shadowed an ER physician, so why not talk to him and see if he knows of any podiatrist that you could shadow for a couple weeks. The thing is that if you don't know this podiatrist that well you will have to shadow most likely more than 30 hours for him to actually write you a decent letter. I have seen people on these forums saying that they shadowed only a couple times and they didn't even know the podiatrist at all. To me, that would be equivalent to asking for a LOR from a college professor in which it was a 200+ student class and you never went to see the teacher during their office hours, but you asked for a LOR just because you got a 4.0. It will be hard for the DPM to "WOW" the admissions committee if the guy barely knows you & he only saw you less than half a dozen times. That is just my personal opinion on Letter of Recommendations.