I keep going to do this too - but haven't got round to it yet! I found this link to a recommended German site via a German hamster owner who made her own mix. It has a recommended mix on it which sounds good to me. I've pasted it below plus the link to the page, if you do select all and copy on the page and paste it into google translate you can read the whole thing.
It does say that the dry mix isn't enough on it's own and that it needs supplementing with fresh veg, herbs, forage etc. It might need supplementing with vitamins and minerals too, unless you can work out the correct balance of required vitamins and minerals in the veg given. I'd say add some cheese once a week too. Also mealworms and other protein like chicken and egg as there doesn't seem to be much protein in the mix.
I think as Cypher says, a mix would need to be complete so that if supplements weren't enough to supplement they could be a bonus.
Recipe for a high-quality dry food hamster:
200 g 5 grains mixture If possible with spelled [I think this means Spelt wheat]. The mixture usually consists of millet, barley, oats, rye and wheat. There should be little wheat be included, who mixes itself, can replace the wheat by Emmer wheat. Finished mixtures are available in the organic section of supermarkets or health food store.
Suitable 100g birdseed / small seeds are sugar-free Budgie Food with little poppy and rice. It should contain various millets and grass and herb seeds. Among the most Budgie Food species grass and herb seeds need to be mixed.
100g multigrain flakes or multigrain cereal flakes are offered because in them the strength is disrupted by heating and squeezing and so can be better utilized. This is important especially for older animals and animals in growth. Well suited are sugar-free muesli with multigrain flakes. Most of these consist of oatmeal, wheat flakes, barley flakes and rye flakes. Even rice flakes may be included. Sugar-free corn flakes (corn flakes) are also allowed and popular. Raisins or other dried fruits should be included but only in small amounts.
100 g dried vegetables carrot chips, parsnips, celery cubes, beetroot cubes etc .. These can be purchased as a finished mixture or individual components in the pet trade.
~ 80 g insects / Eiweisfutter There are several ways again. In the Terraristikabteilung in pet shops or at Rodipet there for. B. dried mealworms, Bachflohkrebse, crickets, Bombyx mori larvae or insect food for small animals. Young animals get more Eiweisfutter, in old animals, it is somewhat reduced.
100 g herbal mixture A quality forage contains Anyway different proportions of dried herbs. These are welcome to be tailored to the taste of the hamster, some may prefer dried flowers like sunflower petals, chamomile flowers, marigold, others may like green crops like green oats, and still others prefer herbs such as dandelion, lemon balm, yarrow and Spitz Weger Rich.
50 g nuts / seeds Sunflower seeds, hazelnuts and walnuts are offered as fat suppliers.
50 g of mixture to make more interesting than treat and to the field pea flakes, corn flakes and rice flakes and dried fruit (if no fruit muesli is used) mixed into the feed.
Please note: this feed mix is not a complete feed! We expect that the additional hamsters get vegetables, forage, fresh Eiweisfutter, herbal hay and other herbs, this was taken into account when compiling the recipes!
The website is supposed to be highly regarded in Germany though.
HAMSTER INFO - Ernährung
Otherwise - you can order Harry Hamster or Burgess Supahamster from Amazon uk.