Tack a few tubes onto your next Ribble Order.

Deda Tubes, $3.11 each
Panaracer tubes, $3.23 to $4.96 each depending on valve

They frequently have sales... right now, 10% off (most items)... which would put the prices at under $3 each, which is less than your bulk pricing.

Just buy enough stuff to hit their minimum order sizes.

Most of the Department stores around here sell tubes for about $5 each... less than your quoted LBS prices, although they seem to never quite stock enough narrow 700c tubes.