I am so sorry about Rookie. She is playing over the rainbow bridge now, free from suffering. There are many people who bury their hamsters without having a garden and use a large pot and plant something in the pot. I know Coco has done this and maybe some other members will come along and suggest the best way to do it.
Otherwise there are pet cremation services and you can pay a bit extra for an individual cremation - otherwise it is a communal cremation, if that makes a difference to you.
Is this place anywhere near you? It sounds nice - they have a pet cemetary for burial as well as a cremation service.
https://www.pcsonline.org.uk/cremato...et-crematorium
It helps when you have had some kind of little ceremony to say goodbye. When our Charlie died we wrapped him in tissue paper and laid him in small cardboard hamster coffin (I was prepared for the inevitable) so he looked peaceful before burial.
There is a thread here with an example of a pot burial which looks lovely and some more info on it on the thread too.
Hamster Burial
I believe you put some stones in the botttom, then a layer of potting mixture then the burial area then fill the pot with the potting mixture and plant something.
I had one of these pebbles for Charlie - it would sit nicely on the top of a pot on the soil.
https://www.ebay.co.uk/itm/Pet-Memorial-Pebble-Stone-effect-Personalised-Freestanding-Weatherproof/111666541457?hash=item19ffd83391:g:lDYAAOSwEeFVTok b:rk:3
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