Several companies sell stiff soled shoes for flat pedals or even (God forbid) clips and straps. Performance and Nashbar will have sales, Van's has some nice ones, as well as Five Ten.
I think SPD style mountain bike shoes are fine for walking. I've been using clipless for more than 20 years, even off-road.
Something like these;
http://www.nashbar.com/bikes/Product...1_10000__11501