Theres currently no such thing as CBD only strain, sadly. But it's coming. In a few years, we'll have them.
The reason we don't have those strains is simple: we didn't know CBD even existed until recently. Cannabis strains that are popular today were bioengineered over the last decades to maximize THC content. Whereas we now have strains like White Russian that have a 22% THC content, twenty years ago, you would have been lucky to get cannabis with even 5% THC.
Now that we know about the usefulness of CBD, many medical growers in Spain and all over Europe and Middle East have put a lot of effort into breeding new strains that seek to maximize CBD content.
That being said, to avoid feeling high, what matters is the ratio of CBD present vs THC. Ideally you want both cannabinoids in high quantities because THC is also very potent when it comes to fighting inflammation.
What you want to look for is strains with a CBD:THC ratio of at least 1:1. Some researchers have determined that a 40:1 ratio (40 times more CBD than THC in a dose) is enough to completely nullify the high.
The best you can find nowadays are 1:1, like CBD Yummy:
http://cbdcrew.org/varieties/cbd-yummy/
Ideally, you want two strains for your medicating.
For the day, you want a sativa dominant strain. Their cannabinoid profile usually induces a more cerebral high that is more conductive with doing chores, work, etc.
For before bed or nights, you want a potent indica strain like Jack Herer, any Kush variant, White Widow, White Russian, etc. These will knock you out more but will also work more aggressively on the inflammation present where we have endocannabinoid receptors.
I'll warn you right away that 1:1 strains can be hard to find. I had to grow my own to get access to them since nobody in Canada grows these. Neither on the black market or the medical market I have access to.
Lastly, your tolerance to the high will build up over time but the medical properties will be retained. Assuming you keep ingesting/vaporizing/smoking the exact same doses every day, eventually, the high will be barely noticeable.