You should be able to find it in just about any garden store. I think Sequestrene is a brand name. If I remember correctly, Miller 138 is another brand name for the same kind of iron chelate. What you want is EDDHA chelated iron (there's a garden store near me that carries a less well known brand that is much less than Millers and Sequestrene).

I checked Lowe's today and all they had was ironite, which I aleady have. The search continues!

Check this list to find the "real" garden center closest to you and call to see if they have it. I'm quite sure Paulino's would have it, but hopefully you'll find it closer.

Skybird

Here is a link that might be useful: Colorado retail garden centers

I'm sorry. I should have been more clear. I was thinking of smaller stores that are specifically garden stores, not the garden centers of the big box stores. I'm thinking more along the lines of a local nursery or independent garden store. Skybird's list is probably a good spot to start.

This google search shows a bunch, also.

Thanks bpgreen and skybird! Picadilly Nursery is about a mile from where I work and I have bought several trees from them. I don't know why I didn't think of it myself. Ah, the joys of the middle aged mind. Thanks again, I appreciate it!