Sometimes you might have a local fish store which sells driftwood without even knowing it!This can be a wonderful solution,if you're lucky.
As for standard pet stores,I know that chain pet stores do in fact tend to sell some safe wood as long as you can find out what kind of wood it is.I've found grape driftwood at Petco,Petsmart,Petsupermarket,etc.If you find Mac's(sometimes Mac's Critter Creations I believe,or something along those lines!)then that would be fine to use as long as it specifically states what wood it is on the tag.Grape driftwood is extremely common,but so is mopani which is unsafe
If local pet stores are possible check there as well,since if they have a basic fish or reptile section they most likely will carry some form of driftwood and that is generally a guarantee that you'll find grape.
If you go to a pet store and find lava rock(fish or reptile section,not the artificially dyed or questionable nonsense in the small animal section),that can be a great option as well since unlike smooth rocks it will actually help to keep nails trimmed.Just make sure to rinse it as good as possible with hot water,and preferably freeze it after for at least two days as with any natural material type of item.