There are Chinese sellers selling the GP3 film in 4x5 and 8x10 sizes.
If you want a cheaper sheet film try FP4, HP5 or Delta 100 which are very good, as good as Kodak in practice. GP3 is very easy to scratch when emulsion is wet, it would be extremly painful to use GP3 if you don't play extreme care. Also latitude, resolving power and grain are not the same compared to ilford/kodak, for sure you may make amazing photographs with GP3, but if you are starting you will probably get better initial results with ilford.
If you want to go GP3 first try it in 120 or in 135, learn how to use it, compare to ilford choices and decide, before doing that I'd prefer ilford sheets. Learn cheap how a film works with 35mm rolls.
I also know the 320 version is more typical for studio use. But how does the look of the two films differ, depending on subject matter? Is it only meant for studio flash use, or can it be used in natural lighting as well? What does it look like if used outdoors? My only other option is to use TMAX 400 which is available in 4x5 and 8x10.
You can use TXP 320 for everything, while TX is long toe TXP has a bit shorter toe (mid size), so the way shadows are compressed is a bit different. Also TXP, (depending on EI and processing) may deliver a bump in midtones that likes portrait photographers.
TXP is a more a classic film with S shape that sometimes may allow an easier to print negative, while TMX delivers a linear negative that is more flexible but sometimes it may command a more elaborated printing technique. In fact the sensitometric footprint of TXP may allow to adjust the tonal scale in the negative while the linear TMX capture delegates the entire tonal manipulation job in the printing process. No way is better than the other... if you are to scan then this is mostly irrelevant, as you easily manipulate the tonal curves in Photoshop.
With Delta 100, HP5 and FP4 you may get similar results than with TMX or TXP, with the right technique.