Didn't you used to be able to buy them from a couple of places in town (Transport Museum and tourist information I think)? Has that been scrapped. If they're not going to have a shop they should at least have somewhere for people to buy tickets.
I purchased a 6 game voucher package a couple of weeks ago and was reliably informed that you could use voucher at turnstyle in exchange for match ticket. I did this for the Crawley game. Operator looked a little puzzled but still allowed me admission, but no ticket in exchange.