One vendor on Amazon has SRAM 11-26 PG950s for sale for $18.
https://www.amazon.com/SRAM-PG950-9-...1-5&th=1&psc=1
I'm not sure you're going to find a new 8-speed cassette much cheaper than that. And I'm pretty sure 11-26 is within what Shimano says the 7700 will handle for a largest sprocket.
Other good 9-speed cassettes can be had in the $30 range.
Personally, I'd use the correct cassette (e.g., 9-speed). I'd probably go with the SRAM - and at that price, I might well order a replacement for when the first wears out.
But it's your money and your bike - as well as your time/trouble/frustration if you want to go the "I'll use mismatched parts and see if they work well enough" route instead.