From academic collaboration to research contract, best practices
The best way is probably to talk very openly with them about your situation and especially what research contracts mean for you. Before you make any suggestions, it would also be good to evaluate the funding situation of your partners, and whether they need your contribution so much for their research that they would be willing to put money in it.

However, most collaborations between academic groups are seen as something that benefits both sides, so it could be seen as a little bit odd that one side pays the other to do something. Still, if your partners have a good funding situation, they might be willing to (co-)fund a PhD student or post-doc in your group, who works specifically on the collaboration project. But I've mostly seen this between groups in the same institution so far, not among different institutions. If your collaborators are in industry, then it will probably be easier.

Another nice way to get such project going would be to apply for joint funding with the other research group, for example within an EU project, and set up a contract how the money from the funded project will be distributed between you and the other group. If your collaborators have good national funding sources, it may also be possible that they apply for funding with the joint project on their own, and outsource the computational part to you.