Some suggestions below for a "proper" liner...however I found some fleece zippered sleeping bags at Walmart for $8.00 that would fit the bill nicely as an add-on to the inside of your sleeping bag (claims to make your sleeping bag's rating temperature drop another 15 degrees). Also just look for a "long" sleeping bag since you are tall. If you do get a different sleeping bag, make sure to get a poly-bag instead of a down bag because of the "moisture" factor. Best of luck from Oklahoma...
P.s. That foam pad is one area where you shouldn't skimp if your going to camp/sleep in the outdoors very often. I balked at the prices of thermarest until my best friend (ice/rock climber and tri-athlete) let me borrow his for a weekend. I ordered one the following monday.
examples:
Liner:
http://www.rei.com/online/store/ProductD...vcat=REI_SEARCHLong Sleeping Bag:
http://www.rei.com/online/store/ProductD...MPING_HIKING_LD 
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