Where are you looking to buy from? It's readily available through internet sales.
RG11 has better transmission characteristics, but it isn't as flexible as RG6 (and on average costs more per foot). Don't have enough info to determine if it is necessary. You should talk with the internet provider directly.
There are two different commonly used types of coaxial cable. One type (including RG-
6), used for CATV, video and most RF purposes has a characteristic impedance of 75 ohms.
The other type, including RG-
58, used for some instrumentation and for thin net Ethernet and has a characteristic impedance of 50 ohms.
If you use a 75 ohm coax for 10BASE2 (thinnet) Ethernet, the termination resistances will be wrong and there will be reflections that will reduce performance.
Both RG-6 and RG-11 are 75 ohm coax, so neither of those are suitable for Ethernet. They are both OK for video and some other forms of data, but the correct termination resistors must be used.
If you use 75 ohm cable with 75 ohm terminations for Ethernet, you will probably be OK, but you will be in technical violation of the standard, and if someone mixes lengths of 50 ohm cable in as patch cables or jumpers you will have serious problems.