There are many Odoo partners with varying price and subscription options. You can go to this link to see them:

 https://www.odoo.com/partners

You can choose one from here, accept a plan feasible for you and ask them to customise. There’s also freelancers who customise Odoo, but their quality may be compromised.

I suggest you choose the number one gold partner, their support and work quality will be the best.

Hope this helps.