Packs of toy beetles can be had on Amazon for 5 bucks or so, such as these:
https://www.amazon.com/U-S-Toy-Bugs-dozen/dp/B00362TS7K/. I think these are the right size, though they need more base-width than 25mm. In the states you can sometimes get them at party supply stores, and for less $.
There are nice toy scorpions as well but they tend to come only one scorpion to a pack of many bugs. One of the nicest is that in the Safari "Venomous Creatures Toob:"
https://www.amazon.com/Safari-Ltd-Venomous-Creatures-TOOB/dp/B0081GG00Y/ref=sr_1_fkmrnull_1?keywords=venomous+toob&qid=1551636091&s=gateway&sr=8-1-fkmrnullI have this set and the other creatures also make great monsters in 28mm, but you would have a hard time finding homes for all the rest if you get enough toobs for a unit of scorpions.
There is this pre-painted D&D beetle that is a nice figure, and sturdier than the above, but needs 2" of base-width:
https://www.coolstuffinc.com/p/125044"Celestial Giant Stag Beetle" from the Blood War set.
There are also several giant scorpions in the D&D sets you may find on ebay, though they might be prohibitively expensive, such as:
"Hellstinger Scorpion"
"Fiendish Monstrous Scorpion"
"Stormclaw Scorpion"
If you stick with a requirement for 25mm base width, your task is going to be a lot more difficult, if not impossible. The sculptor basically has to fudge the anatomy the way GW did for the forest goblin spider-mounts (both metal and plastic).