If you are providing this as a service, you should be providing the strap-on as well. There's a couple of good reasons for this. First it means you can select a harness that you both enjoy and fits you well. Most ladies who enjoy giving strap-on will tell you that a good fitting harness is a must for a good session. Having it move all over, chafing of your nether bits (ouch), or having it fall out of the holder because its not a good fit (how embarrasing!) is best avoided.
You'll also have more control over the sizes and shapes of your dildoes. This will allow you to scale up your penetrator from 'virginal' to 'i can fit a mack truck in this ass!'.
Plus, well, as the provider, it is kind of incumbent upon you to provide this. If you were to go to a restaurant, you would expect them to provide the steak, right?