Do you by chance have a TV antenna on your roof? You wrote that you listen to 700/800 mHz and UHF/VHF. TV antennas are very good at that and inexpensive. And no installation necessary if you already have one! You'll need an adapter to go from the TV antenna line/coax to your scanner. But that's easy to do compared to going on the roof. If it works, you can make it even better by turning the antenna to make all the elements (pointy rods) vertical instead of parallel to the ground.
The drawback to the TV antenna is that most of them are directional. If the stuff you are trying to listen to is in the direction the antenna is pointing, then you're in luck. If not, you'll need to rotate the antenna to point toward the area you want to receive.
You can buy special antennas for scanners that receive in all directions. But if you do that you must absolutely:
A - Spend more money
B - Get on the roof
C - Find a mounting location and necessary brackets, etc.
D - Drill holes, run cable, install lightning protection, etc.
If you want a special scanner antenna and are OK with doing A, B, C, and D.... then check the base scanner antenna listings here:
Scanner Antennas - The RadioReference Wiki
There is also one other place that is popular here on RR that I don't see in the Wiki:
http://www.theantennafarm.com/catalog/antennas-191/
When you get to that page click on "Base Antennas".