Used to be sold already on a trellis by big growers such as Monrovia, who truck to garden centers over a large area. Try their web site, if they are still producing them they have a search function on their web site that helps you locate their customers.

When telephoning local retailers, ask for a camellia on a trellis in case espalier doesn't mean anything to who you happen to get on the phone.

I agree with Ron. A classy garden center will carry those items typically. This might not be the best time of year to find them, though. Garden centers tend to invest in the pricy items at their most sellable time...which would be during the flowering time for camellias.