Hamsters and rats are both omnivores so both foods would be accepted. However, in turn to guarantee the best nutrition for your hamster, you should feed hamster food. The top premium ingredients in these 2 foods are different, meaning they'll result in different things. Hamsters don't need daily fish oil supplements, for example. Hamsters NEED to forage for different seeds and treats.
I wouldn't get the hamster Oxbow if it's too hard to get. Lab blocks shouldn't make up their main diet anyway. I prefer to scatter seeds and things around for them to scavenge (naturally) for. That said, if you did buy an entire bag of rat blocks, the hamster may never go through them all (since you would only feed it alongside an actual hamster food to ensure complete nutrition).
So I wouldn't get the rat blocks either,
Does any of your stores order specific customer request?