You can go to Home Depot and buy the Citripots (4"x4"x16" rectangular pot), or to your local nursery. These are cheap, less than $12. Select the orange types with good stems and showing vigor. These are excellent to graft to, and very quickly too. This is by far the most convenient if someone is in a hurry, and for sure you will already have an extra cultivar in there. Most Citripots are on standard or semi-dwarfing rootstocks, and these trees grow much quicker than those in the 5-gallon pots if you plant them inground at the same time.
But if you want long term cheaper solution, you can talk to store manager of local nurseries in your area and you might be able to get rootstocks on the cheap. There are also rootstocks available from
http://www.citrustreesource.com/pricing/
But usually, these less than $1 rootstock seedlings are not ready for grafting once you get them. But they can be grafted if you know more advanced micrografting techniques.
T-budding would be my most preferred method on smaller citrus trees.
I'd still go with buying a citripot from HD to quicky transfer and propagate a souvenir DNA from the existing tree. It is the quickest and most convenient, and for that, you pay some money for it.