Good recommendations Shailesh. Alibaba is a good source, but it really depends on volumes. If your only looking at hundreds of units then the considerations are much different than if you're looking for thousands or even larger volumes. It's a tradeoff between the cost benefits of volume purchasing vs. the support you require. If you're looking for small volumes forget about trying to engage a large player (they won't be interested in supporting) you, if the volumes are small but the changes you require significant then look for a smaller player perhaps of custom or specialty products. Your unit cost will go up significantly but your level of support will as well. And, in some applications the unit cost isn't the driving factor (if you're charging the client significantly for your value-add, perhaps the hardware cost is a small part of the overall costs?). As in everything the optimal solution will depend on your constraints.
Are there specific custom hardware requirements for this device or is it purely custom software sitting on an off-the-shelf tablet (or the equivalent)?
What kinds of volumes are you looking for? If you're looking for small volumes then I think you're at the mercy of what you can find on-the-shelf. If you're looking for larger volumes then you can interest manufacturers, perhaps even into providing the changes or build you're looking for.