How much do you lack? If you're only an inch or so you'd come out cheaper getting wheel spacers. If it's like 3" you may need wheels with more backspace
Good question. How can I measure the needed offset to clear the frame? Loosen lug nuts and pull wheel out, try and measure gap needed to clear at turn?
take the wheel off and measure from center inside of rim to flange at edge = backspace
knowing what rim offset you have, as well as factory backspacing, will tell you how much spacer needed.
spacers won't go thru most state inspection, fwiw
nice thing about spacers is you can use with multiple rim sets/rigs (if you have more than one each, obviously)
If you have to buy them, spend the extra few dollars and get rims...
spacers always come up for sale used, rims not as much, and usually come w/tires you may or may not need/want