Is there fossil evidence (of transitional features) in support of the evolution of frogs from non-frogs? Yes!
See https://en.wikipedia.org/wiki/Frog . In part it says the following.
"In 2008, Gerobatrachus hottoni, a temnospondyl with many frog- and salamander-like characteristics, was discovered in Texas. It dated back 290 million years and was hailed as a missing link, a stem batrachian close to the common ancestor of frogs and salamanders, consistent with the widely accepted hypothesis that frogs and salamanders are more closely related to each other (forming a clade called Batrachia) than they are to caecilians.[25][26] However, others have suggested that Gerobatrachus hottoni was only a dissorophoid temnospondyl unrelated to extant amphibians.[27]
... The earliest known amphibians that were more closely related to frogs than to salamanders are Triadobatrachus massinoti, from the early Triassic period of Madagascar (about 250 million years ago), and Czatkobatrachus polonicus, from the Early Triassic of Poland (about the same age as Triadobatrachus).[28] The skull of Triadobatrachus
is frog-like, being broad with large eye sockets, but the fossil has
features diverging from modern frogs. These include a longer body with
more vertebrae.
The tail has separate vertebrae unlike the fused urostyle or coccyx in
modern frogs. The tibia and fibula bones are also separate, making it
probable that Triadobatrachus was not an efficient leaper.[28]
Be sure to visit the above mentioned web page of https://en.wikipedia.org/wiki/Gerobatrachus_hottoni . See also https://en.wikipedia.org/wiki/Triadobatrachus . The latter says in part the following.
"Triadobatrachus was 10 cm (3.9 in) long, and still retained many primitive characteristics, such as possessing at least 26 vertebrae, where modern frogs have only four to nine." It also says "It lived during the Early Triassic about 250 million years ago, in what is now Madagascar." It also says "It was first discovered in the 1930s ...."