The red bulb can be used both durning the day and night. And, you can just get one from Home Depot/hardware store and it will work just as well as the ones from the pet stores.
You might want to look into getting a dimmer switch for your UTH so that you can control the amount of current going to it, and thus controllling the temp. Use a thermometer to find the correct temp as you adjust the dimmer switch, and then just leave the switch as that setting for the temp that you want.