I give my hamster a seed spray about once a week. Usually she plucks it bare during the first night but then she doesn't get another one until next week.
I buy the Rosewood Harvest Festival pack of seed sprays on
Amazon which comes with dari and flax, and I also buy millet sprays from my local pet shop. Rosewood also do lots of nice herb mixes on Amazon (search for "rosewood naturals forage").
I stick the sprays upright in the little wooden stumps you can get from Pets at Home. Below is actually my gerbil cage, not my hamster cage but I do the same thing in both cages:
