Go to Florida twice a year.
Rock & Roll Heaven in Orland worth a look. Usually pick up a few Motown bits but nothing special, although on my last visit found a Flrtations "Nothing But A Heartache" on Deram, mint copy only $4!
They have got boxes of records under a counter which haven't been sorted yet. There are some soul records in there but they're in artist order, as far as I remember but would imagine anything of value has been pulled and listed on ebay but you never know if you're prepared to do the leg work and sift through them.
Found a few bits at a local flea market, including a Jackie Wilson "I've Lost You" for $1.
There's another record shop not too far from R&R Heaven, can't remember the name, loads of LP's but absolute zilch soul 45's.
Done lots of local towns thrift stores (2nd hand shops) but most of em don't have any 45's at all.
If you're staying in Sarasota, Tampa might be worth a visit.
Miami - I haven't look for records yet but based on that city's soul input (70's especially) I'd be surprised if there's nothing there.
Mainly, however, I suspect that Florida's already been done big time by visiting Brits and the only way you might get lucky might be at a flea market, garage/yard sale when someone on the day just might be selling a collection