I just ordered a SEF butterfly search coil for my ace 250. As of right now the only place you can get them is from the UK, although there are companies here in the US that sell these coils for other brands like Whites and Minelab. I was pretty scared about doing this but am now very glad I took the chance. It cost me just under $300 and it's 11 x 14. It only took about 2 weeks to get it which shocked me. I have been using it now for about 3 weeks and love it as I have had much success. The only complaints that I have is that it's very heavy and leaves your wrist a little sore, it might help if I used the arm strap but I never do. Also that pin pointing is not always very accurate. I do not work for any company related to metal detecting what so ever, I work for an insurance company. I have just tried to do as much research on these before I purchased mine. I hope this helps.
One last thing, I think the companies here in the US should add these to their inventory as there are more people with Ace 250's then any other that I know of because of the affordable price and quality which would boost the sales (in my opinion).