If you have good soil, you don't need them, as they'll be there anyway. If you have poor soil, you don't need to grow roses until it's improved. Stay away from too much high P fertilizers and avoid overusing the manure and you'll have them naturally in your garden.

As HollySprings suggests, if there is much phosphate in the soil, the fungi don't colonize roots. I'm afraid most rose beds already have too much P from manure, bone meal, or superphosphate to get much benefit from mycorrhizae. Conversely, if there are mycorrhizae, very little phosphate is needed for good growth.

I read the other day that the reason phosphate boosts root growth is that it makes the roots less efficient by excluding the mycorrhizae-- so the plant needs to put more energy into root development.

Mychorizal fungi are fairly plant specific so its doubtful that what you would "buy" would be what you would need. If you get your soil into good health, add lots of organic matter, and don't disturb the soil those fungi will populate your soil without you doing anything much, so save your cash and use it for something else, such as more plants.

There are two main types of mycorrhizal fungi: Endo and Ecto. Endo Mycorrhizal fungi colonize around 85% of all the plants in the world, whereas Ecto only colonizes a few trees and shrubs. There are a handful of plants that require specific types, but roses will form a relationship with Endo. If you plan to purchase, make sure that you know the exact spore count, and not the "propagule" or "viable organism" count. Most of these companies use chopped or ground up roots, but you can find one that has the technology to sell just the spores.
Most of todays soils are lacking in beneficial fungi and bacteria. So unless you are getting your soil from an undisturbed forest, you may want to add your own.

roots,

It's true that you can't be sure unless you inoculate. But I found this statement in a technical report: "cultivated rose is normally mycorrhizal." I would guess that grafted roses normally pick up fungal partners in the rose fields and bring them into our gardens. Own-root greenhouse roses from the boutique rose nurseries would lack them and would probably benefit from inoculation if they are to be grown in pots.