Ok, I know this has been asked before, so here is your answer because I just found out.
Dunlop human heating pad, kept on most of the time, and occassionally heat setting was changed to compensate for heat levels too high, it lasted me 3 years 5 months. I am happy with that. Pretty darn great if you ask me, considering it was only a $10 investment.
Prior to using that heat pad I used one of those ones for reptiles you stick on the bottom of the cage- it last 3 months. Not worth my $ ever again if you ask me, considering the reptile one also had hot spots in it.
And- yes Walmart dont carry the human heat pads with no auto off. So, I went to several local drug stores last night in search of my new heat pad. Went to CVS, RiteAid, Walgreens and Drug Mart (all for info reasons). They all carry heat pads without auto off for $10. So, keep in mind, that might be where you have to get your next heat pads. However, I could not find dunlop ones, I did however find a brand, that looks identicle to a dunlop I have under my boa cage, looks like it was made by the same place, just a different name sticker slapped on it. The company is Kaz.