Just measure the existing bolts and the thickness of the flange on the brackets. Get bolts that much longer. Use grade 8 bolts.
Any decent hardware store, Lowe's, Home Depot, etc, should have them. Probably no more than $12-15, if that much. Use the existing nuts/lock washers or new nylock nuts.
You should be able to remove one by and take it to the store with you to check thread pitch, length, etc.
Don't be surprised if there are two different lengths for upper and lower mounts.