Amazon has them, read reviews carefully though, some are next to useless. I got some last year as my dog kept getting urine infections, I’ve yet to use them as she hasn’t had one for over a year.
I used some recently. They never showed ovulation. I even e-mailed photos to the seller and was told that no she isn't ovulating yet, it will be several days until she is. I decided to go on the behaviour of the bitch as I've always done in the past and she was mated despite the negative results and is now pregnant with a large litter. Yet other people swear by them.