Hmmm...don't know of any in the Myrtle Beach area, but depending on which way you drive down you might pass some good ones along the way. Ones I particularly like:
Owens Orchids is near Asheville, about 30 minutes off I-26 in Pisgah Forest. Doesn't seem very well-known anymore, but has a really nice selection of Phals and Catts, plus a bunch of more obscure stuff if you poke around in the back.
Marble Branch Farms is in Walhalla, SC, probably about an hour side-trip each way from I-26, but WELL worth your time if you like Cattleya species and hybrids. Amazing prices and very nice proprietors; just make sure you make an appointment before heading over.
Carter and Holmes is further down I-26 in Newberry, SC, IIRC only about 10-15 minutes off the interstate. This place is huge, plan for a few hours if you stop there!
Carolina Orchids is about 3 minutes off I-77 just over the state line from Charlotte. Woodland Orchids is also in the Charlotte area, but I can't remember their proximity to I-77 (probably not too far, though). Carolina has one of the bigger inventories of Bulbophyllums in the US, plus a lot of other hard-to-find things in various genera. Woodland seems to specialize in Oncidiums and has a lot of nice plants as well.
Orchid Trail is right near the Raleigh-Durham airport, I think only about 10 minutes or so off I-40. I've only been there once, but they had a lot of unusual plants and some really great deals on supplies (18x36" treefern slabs for $8!).
Hope this helps, Amber!
--Nat