There is a MyTarps.com in Marietta, GA where I have purchased tarps before including a very large one to cover my workshop roof while under construction.

Here are some examples:

Sigman 15' x 30' Blue Tarp - Finished Size
MSRP: $69.99
Price: $24.75
You Save: $45.24 (65 %)

15' x 25' Green Silver Tarp
MSRP: $103.99
Price: $42.56
You Save: $61.43 (59 %)
Sigman 12' x 25' White Tarp Heavy Duty - Finished Size
MSRP: $119.99
Price: $37.85
You Save: $82.14 (68 %)

Sigman 18' x 24' White Tarp Heavy Duty - Finished Size
MSRP: $172.99
Price: $53.62
You Save: $119.37 (69 %)
Sigman 15' x 30' White Tarp Heavy Duty - Finished Size
MSRP: $179.99
Price: $58.00
You Save: $121.99 (68 %)

Many different types and duty ratings.  I also agree that a cover for the existing 2 eating canopy skeletons is much cheaper than something new.
Just figure out the size and type that would work best, and get some bungee cords.

Bob
K4VBM