You can buy it at convenience stores, supermarkets and bakeries very easy.
Bakery's ones are a bit expensive, but the bread's dough are different and more tasty.
High school students eat cheapest one sold at convenience stores and supermarkets,
but they also tasty, although it may destroy yr dream, it is not melon taste bread.https://en.wikipedia.org/wiki/Melonpan
These are not latest ones, but you can see the price, 100Y one is 108Y(sales tax 8%).https://mognavi.jp/news/tabekurabe/69289/http://mediapush-push.com/wp-content/uploads/2016/12/img_8861.jpghttp://portal.nifty.com/2006/06/19/c/
Melon taste melon pans are also sold, but it will be difficult to find in Osaka, this type one is rare. https://b-lunch.com/ebina-sa-portugal-melonpan.html
Not only in Osaka, but also in big cities,
there are specialty shops that sell only melon pans like this video, this bakery is in Osaka though.https://youtu.be/Jf3vUFH_vj0
Other foods also same to find,
"Taiyaki(たい焼き)" and "Dora yaki(どら焼き)" have also high-class ones,
but it is not difficult to find cheap ones as street food.
High-class ones are different, high-end "bean paste(餡：あん)" using.
Except Mitarashi dango, there are many variants with "different foodstuffs",
those are called "変り種(kawari-dane)", 種(tane) means "seed" mainly but also means "core".
I do not know if you've heard this word, "neta(ネタ)" of sushi(寿司/すし) is a word
which became to be used because this "tane" was read in reverse.
"Neta" as a food ingredient is basically not used except for sushi.
In cake making use correct reading "tane".https://en.wikipedia.org/wiki/Sushi
This "neta" is not just used for sushi,
it is one of the Japanese that you use everyday like "theme", "idea","topic" and more.
Because there is no variation in taste of Mitarashi dango,
it would be better to buy and eat at Osaka's traditional shopping street(商店街).
If you go to the ancient capital of Kyoto or Nara,
the tea house(茶屋, classical style cafe) near the shrine
and similar style cafe featuring Japanese-style sweets(甘味処) would have a feeling.http://naratokidoki.seesaa.net/article/388801120.htmlhttp://rokuhoudou-anime.jp/
In 甘味処, "anmitsu(あんみつ)" and "Zenzai(ぜんざい)" are popular among women, also ice cream(アイスクリーム) and parfait(パフェ), in Kyoto and around "Matcha(抹茶)" using menu is usual.https://bessynara.com/bunnosuke-kyoubasi/https://news.milize.com/2014/08/7257/https://en.wikipedia.org/wiki/Shirukohttps://en.wikipedia.org/wiki/Matcha
Anyway, there are many other famous foods in Osaka.
Takoyaki(たこ焼き) and Okonomiyaki(お好み焼き) are also representatives of them.https://en.wikipedia.org/wiki/Takoyakihttps://en.wikipedia.org/wiki/Okonomiyakihttps://www.japan-guide.com/e/e4022.htmlhttps://migrationology.com/osaka-food-guide-japan/
Although this post does not mention specific foods,
the diversity of Osaka's food culture is concentrated in Japanese word "kuidaore".http://vivianlostinseoul.blogspot.com/2011/06/dotonbori-and-kuidaore.h...https://www.instagram.com/explore/tags/kuidaore/https://en.wikipedia.org/wiki/D%C5%8Dtonbori
It is full of temptation that want to cancel music CDs plan to buy in Osaka.
CD can be bought at amazon or iTunes as music files,
but real Japanese food can only be experienced in Japan.
Their lang are neither Japanese nor English,
but they introduce a lot of things about Osaka and nearby Kyoto and more.https://www.youtube.com/user/xpcubex/videos?disable_polymer=1